Risk factors for obstetric anal sphincter injury in a Norwegian Region: a cohort study

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Well-known risk factors for obstetric anal sphincter injury include primiparity, high maternal age, instrumental delivery, oxytocin augmentation, and large fetal size. Recently, we identified amniotomy as a potential novel independent risk factor, regardless of parity. This study aims to further explore whether amniotomy increases the risk of obstetric anal sphincter injury in singleton vaginal births after 30 weeks of gestation at Innlandet Hospital Trust, using a cohort study design. Methods Retrospective cohort study using data from the Innlandet Perinatal Database, including all women who had singleton vaginal births after 30 weeks of gestation at Innlandet Hospital Trust from 2012 to 2022. We used multivariate logistic regression models to develop a risk-factor model for obstetric anal sphincter injury employing backward elimination with stratification for parity. Results The annual incidence of obstetric anal sphincter injury at the hospitals was stable around 2% in the study period and a total of 26349 births were included. Amniotomy was not found to be a risk factor for obstetric anal sphincter injury in this contemporary cohort. Overall, instrumental delivery was identified as an independent modifiable risk factor for obstetric anal sphincter injury: vacuum extraction presenting relative risks of 2.72 and 3.29 in primiparous and parous women and forceps 5.97 in primiparous women. Episiotomy and epidural analgesia were associated with reduced risks of obstetric anal sphincter injury, but only in primiparous women (relative risks: 0.53 and 0.69). Conclusions Based on systematically collected data from our regional birth registry, this cohort study confirmed the association between established risk factors for obstetric anal sphincter injury, such as instrumental vaginal delivery. However, we could not identify amniotomy as a risk factor for obstetric anal sphincter injury, possibly due to changes in clinical practice and potential data limitations. OASI amniotomy instrumental delivery risk factor parity Figures Figure 1 Figure 2 Introduction Obstetric anal sphincter injury (OASI) is the term used to describe the unintentional tearing of the anal sphincter and/or the rectal mucosa at delivery. It is a feared complication of vaginal delivery because it carries a high risk of morbidity for affected women, with both Norwegian and international studies showing 40–50% suffering from some degree of persisting anal incontinence despite adequate suturing. 1 – 3 Internationally, OASI has an incidence of about 3–7% in women giving vaginal birth. 4 In Norway, there was an increase in the reported number of OASI from less than 1% in the late 1960s to 4,3% in 2004. 5 This caused the Norwegian Board of Health to commission an interventional study, with promising results leading to the implementation of a national prevention program aiming to reduce the incidence of OASI. 6 A key feature of this program was manual support of the perineum. In line with the results of the interventional study, a significant decrease in OASI was seen after the intervention, with rates dropping as low as 1–2%. 5, 6 Rates of OASI have remained low in Norway and in 2022, OASI occurred in 1,8% of all vaginal births in Innlandet Hospital Trust, and in 1,6% of all vaginal births in Norway. 7 Primiparity, high maternal age, instrumental vaginal delivery, augmentation with oxytocin, and a large fetus are some of the well-established risk factors. 4 In addition, we recently demonstrated that the risk of OASI was significantly higher in women where amniotomy was performed, regardless of parity. 8 This could mean that amniotomy may be a hitherto overlooked significant independent modifiable risk factor for OASI regardless of parity. Amniotomy is a readily used procedure in the management of vaginal birth to shorten the length of labor by speeding up uterine contractions. However, the use of amniotomy in birth management is debated and most likely varies greatly both between as well as within institutions. 9 , 10 In an updated systematic search of published studies, we have not found any other studies investigating amniotomy as a risk factor for OASI. The aim of this study is therefore to explore if amniotomy can be verified as a risk factor for OASI, in women giving vaginal birth in singleton pregnancies after 30 weeks of gestation in the Innlandet Hospital Trust (IHT), using a cohort study design including contemporary data from our regional birth registry. Material and methods Study design and population We conducted a retrospective cohort study using data from the Innlandet Perinatal Database (IPD). At IHT, detailed information regarding maternal health, pregnancy, delivery, and the early postpartum period is recorded in a digital medical record. Midwives and doctors input data throughout the pregnancy. This structured data is directly transferred to the IPD without modification. Data extraction from IPD for this study was performed retrospectively in September 2023 (see Table 1). Innlandet County, located in eastern Norway, has a population of 371,385 (as of January 1, 2020) with most births occurring within IHT. In the 10-year study period from 2012–2022, 30,445 births were registered in IPD. The study population includes all women who had a singleton vaginal birth after 30 weeks of gestation at Innlandet Hospital Trust from 2012 to 2022. This includes the hospitals in Lillehammer, Gjøvik, Elverum and Tynset for the whole study period, and the hospital in Kongsvinger from 2012 to 2018 (before its transfer to the neighboring Hospital Trust from 2019). Lillehammer Hospital has the biggest obstetric department, followed by Elverum and Gjøvik, while Tynset operates a small midwifery-led delivery ward with approximately 50 births per year. Definitions The outcome variable OASI was defined according to the ICD-10 classification as perineal tears grade 3, involving the external (3A and B) and internal (3C) anal sphincter (O70.2) or grade 4, where the anal or rectal mucosa is involved in addition to the external and internal sphincter (O70.3). 11 The diagnosis was made at the time of delivery, confirmed by the on-call specialist in obstetrics and gynecology. We included the following independent non-modifiable and modifiable variables that could affect the birth process. Non-modifiable variables included birthweight (grams and quartiles), head circumference (cm), gestational age (GA) (weeks and categories), maternal age (years and categories), duration of the second stage of labor ( 60 min), fetal presentation (occiput anterior, occiput posterior, breech, other vertex presentations), and former OASI. Modifiable variables included induction of labor (yes/no), amniotomy (yes/no), use of oxytocin (yes/no), augmentation (yes/no), induction (yes/no), episiotomy (yes/no), maternal birth position (supine/sitting, side bearing, kneeling/all fours, squatting, standing, water, outside of hospital - unplanned), instrumental delivery by vacuum extraction (yes/no) or forceps (yes/no), and finally modes of pain relief; epidural (yes/no), nitrous oxide inhalation (yes/no), or pudendal nerve block (yes/no). For most of the population, GA was estimated according to routine ultrasonography at 18–20 weeks of gestation. At birth, weight and head circumference were measured according to protocol. Induction of labor included transcervical Foley catheter, vaginal or oral prostaglandin, amniotomy or/and intravenous oxytocin. Amniotomy was also performed in births where continuous surveillance of the fetus with an internal electrode was indicated or in the case of labor dystocia, often in combination with augmentation with intravenous oxytocin. Instrumental vaginal delivery, vacuum extraction or forceps, was performed in the case of signs of fetal asphyxia or dystocia of the second stage of labor. Episiotomy was always mediolateral. Indications for episiotomy were instrumental vaginal delivery, breech delivery, former OASI, suspected fetal asphyxia or if risk of perineal rupture during delivery was imminent, at the discretion of the clinician. The duration of the second stage of labor was routinely recorded immediately after birth. Statistical analysis For descriptive statistical analyses, continuous variables were described by means and standard deviations. Categorical variables were described by frequencies and proportions. Separate analyses were performed for the primiparous and parous (one or more previous deliveries) women. The data were quality-assured and to some degree expanded by going through relevant delivery protocols. A p-value < 0,05 was chosen as the level of significance. The included modifiable and non-modifiable independent variables were assessed with univariate regression analyses to explore the associations between the variables and risk of OASI. We then developed a risk-factor model for OASI using multivariate logistic regression with a stepwise procedure, implemented through generalized linear models in Stata. We specified the model with a binomial family and a logit link to estimate risk ratios. In each step of the procedure, we used backward elimination; the variable with the least statistical significance was removed until all remaining exposure variables showed a significant association with OASI. STATA 18.0 software (StataCorp, College Station, TX) was used for all the analyses. Results In the decade from 2012 to 2022 there were 33,572 births registered in IHT. In the analyses, we included all singleton vaginal births in the period from 2012 to 2022 and excluded births by caesarean section, twin births, and births before gestational week 30. This resulted in a total number of 26,349 included births (Fig. 1 ). The yearly total incidence of OASI in IHT has been around 2% for the duration of the defined period of our study. The ratio of OASI in women with spontaneous delivery is lower than in the group with instrumental vaginal delivery (vacuum and forceps). (Fig. 2 ). The annual number of women giving birth during the study period has decreased gradually from 2,934 births in 2012 to 2,482 births in 2022, a 18% decline in the total number of births. Table 1a and 1b describes the modifiable and non-modifiable factors for primiparous and parous women with and without OASI. There were 10,544 primiparous and 15,343 parous women that met the inclusion criteria, the incidence of OASI was 2.9% in the primiparous group and 1.0% in the parous group. Table 1a Study characteristics of included primiparous women Primiparous women no OASI OASI mean (SD) or n (%) mean (SD) or n (%) p-value N 10544 (97.1) 314 (2.9) Non-modifiable variables Maternal age (years) 27.8 (4.7) 29.3 (4.8) < 0.001 Maternal age category < 0.001 35 years 898 (8.5) 48 (15.3) Gestational age (weeks) 39.6 (1.6) 40.0 (1.3) < 0.001 Gestational age categories 0.004 at term (37-42 weeks) 9335 (88.5) 278 (88.5) preterm (42 weeks) 785 (7.4) 33 (10.5) Fetal presentation < 0.001 occiput anterior 9662 (91.7) 262 (83.4) occiput posterior 727 (6.9) 48 (15.3) breech 83 (0.8) 1 (0.3) other vertex presentations 60 (0.6) 3 (1.0) Duration of 2nd stage 0.04 ≤ 60 min 8291 (80.4) 236 (75.6) >60 min 2022 (19.6) 76 (24.4) Weight (g) 3442.1 (472.2) 3639.8 (451.4) < 0.001 Birthweight quartiles < 0.001 3866 g 1854 (17.6) 94 (29.9) Head circumference (cm) 34.8 (1.5) 35.3 (1.5) < 0.001 Modifiable variables Induction of labor 2433 (23.1) 91 (29.0) 0.02 Amniotomy 4040 (39.1) 120 (38.2) 0.7 Oxytocin (augmentation or induction) 5455 (54.4) 194 (63.8) 0.001 Maternal birth position 0.2 supine/sitting 8855 (85.9) 269 (85.9) side bearing 885 (8.6) 26 (8.3) kneeling/all fours 364 (3.5) 9 (2.9) squatting 26 (0.3) 0 (0.0) standing 36 (0.3) 2 (0.6) water 131 (1.3) 5 (1.6) outside hospital, unplanned 11 (0.1) 2 (0.6) Pain relief Epidural 4814 (45.7) 132 (42.0) 0.2 Nitrous oxide inhalation 7768 (73.7) 235 (74.8) 0.6 Pudendal 177 (1.7) 16 (5.1) < 0.001 Instrumental delivery Vacuum extraction 1866 (17.7) 101 (32.2) < 0.001 Forceps 327 (3.1) 35 (11.1) < 0.001 Episiotomy 3944 (39.3) 133 (42.4) 0.3 Table 1b Study characteristics of included parous women Parous women no OASI OASI mean (SD) or n (%) mean (SD) or n (%) p-value N 15,343 (99.0) 148 (1.0) Non-modifiable variables Maternal age (years) 31.4 (4.6) 31.5 (4.3) 0.6 Maternal age category 0.96 35 years 3,795 (24.7) 35 (23.6) Former OASI 127 (0.8) 4 (2.7) 0.01 Gestational age (weeks) 39.6 (1.4) 39.8 (1.3) 0.1 Gestational age categories 0.004 at term (37-42 weeks) 14,215 (92.6) 132 (89.2) preterm (42 weeks) 729 (4.8) 15 (10.1) Fetal presentation 0.4 occiput anterior 14,116 (92.1) 133 (89.9) occiput posterior 1047 (6.8) 14 (9.5) breech 102 (0.7) 0 (0.0) other vertex presentations 60 (0.4) 1 (0.7) Duration of 2nd stage 60 min 348 (2.3) 14 (9.5) Weight (g) 3,621.9 (477.5) 3,825.2 (451.6) < 0.001 Birthweight quartiles < 0.001 3866 g 4,565 (29.8) 71 (48.0) Head circumference (cm) 35.2 (1.4) 35.6 (1.4) 0.001 Modifiable variables Induction of labor 3,270 (21.3) 44 (29.7) 0.01 Amniotomy 5,595 (37.1) 57 (38.5) 0.7 Oxytocin (augmentation or induction) 2,832 (20.2) 60 (42.9) < 0.001 Maternal birth position 0.001 supine/sitting 11,001 (74.1) 120 (81.1) side bearing 2,093 (14.1) 12 (8.1) kneeling/all fours 967 (6.5) 7 (4.7) squatting 32 (0.2) 2 (1.4) standing 104 (0.7) 3 (2.0) water 499 (3.4) 1 (0.7) outside hospital, unplanned 156 (1.1) 3 (2.0) Pain relief Epidural 2,891 (18.8) 52 (35.1) < 0.001 Nitrous oxide inhalation 9,311 (60.7) 88 (59.5) 0.8 Pudendal 78 (0.5) 6 (4.1) < 0.001 Instrumental delivery Vacuum extraction 590 (3.8) 27 (18.2) < 0.001 Forceps 119 (0.8) 4 (2.7) 0.009 Episiotomy 1,361 (9.6) 32 (21.6) < 0.001 Findings for primiparous women (Table 1a) Women with OASI were, on average, 1.5 years older and more likely to have post-term pregnancies. Their infants had higher weights and larger head circumferences. OASI was more frequent when the second stage of labor exceeded 60 minutes, with occiput posterior positioning, and in cases of labor induction, oxytocin use and instrumental vaginal delivery. Epidural use was higher in women without OASI. Findings for parous women (Table 1b) Similar to primiparous women, parous women with OASI had infants with higher mean birthweight, larger mean head circumferences, a higher proportion with more than 60 min duration of second-stage of labor, a higher proportion of infants with occiput posterior presentation, and a higher proportion of post-term pregnancies. Further, among women with OASI, a higher proportion had labor induction, use of oxytocin, instrumental vaginal deliveries, epidural, and episiotomies. The multivariate logistic regression analysis revealed that among primiparous women, an increase in infant birthweight, per 100 g, was associated with an increased risk of OASI with a relative risk (RR) of 1.09 (95% CI 1.07–1.12 [Table 2 ]). Additionally, increasing maternal age was associated with an increase in risk of OASI in this group, one year increase in age gives an increase in RR of 1.06 (95% CI 1.03–1.08). Among the modifiable variables, having an instrumental delivery was associated with an increase in risk of OASI, for both vacuum (RR 2.72, 95% CI 2.06–3.62) and forceps delivery (RR 5.67, 95% CI 3.96-9.00) when compared to spontaneous vaginal delivery. Furthermore, having an infant in occiput posterior presentation was associated with an increased risk of OASI when compared to an infant with occiput anterior presentation (RR 1.89, 95% CI 1.40–2.54). Having an episiotomy or an epidural analgesia was associated with a decreased risk of OASI among primiparous with RR of 0.53 (95% CI 0.40–0.70) and RR of 0.69 (95% CI 0.55–0.86), respectively. Table 2 Multivariate logistic regression analysis for OASI among primiparous women, N = 10,310 Crude Adjusted Variable n RR 95% CI n RR 95% CI Non-modifiable variables Maternal age, years 10,858 1.06 1.04–1.09 10,310 1.06 1.03–1.08 Fetal presentation occiput anterior 9,924 Reference 9,440 Reference occiput posterior 775 2.35 1.74–3.16 727 1.89 1.40–2.54 breech 84 0.45 0.06–3.18 83 0.49 0.07–3.40 other vertex presentations 63 1.80 0.59–5.48 60 1.61 0.53–4.84 Infant birthweight, per 100 g 10,840 1.09 1.07–1.12 10,310 1.09 1.07–1.12 Modifiable variables Epidural 4,946 0.87 0.70–1.08 4,720 0.69 0.55–0.86 Vacuum extraction 1,967 2.14 1.70–2.70 1,961 2.72 2.06–3.61 Forceps 362 3.64 2.60–5.09 362 5.97 3.96–9.00 Episiotomy 4,077 1.13 0.91–1.41 4,063 0.53 0.40–0.70 For parous women, former OASI and an increase in infant birthweight were associated with an increased risk of OASI with RR of 3.68 (95% CI 1.40–9.67) and 1.08 (95% CI 1.05–1.12), respectively (Table 3 ). A 100 g increase in infant birthweight was estimated to increase the risk of OASI by 8%. Among the modifiable variables, having a vacuum extraction was associated with an increased risk of OASI when compared to spontaneous vaginal delivery with a RR of 3.29 (95% CI 2.09–5.17). If the mother had a squatting (RR 8.78, 95% CI 2.29–33.5) or standing birth position (RR 3.45, 95% CI 1.13–10.5) when compared to a supine position, was also associated with an increase in risk of OASI. Having an unplanned out-of-hospital birth or having received oxytocin infusion also increased the risk of OASI with RR of 4.38 (95% CI 1.40–13.7) and 2.00 (95% CI 1.39–2.88), respectively. Table 3 Multivariate logistic regression analysis for OASI among multiparous women. N = 13,358 Crude Adjusted Variable n RR 95% CI n RR 95% CI Non-modifiable variables Former OASI 131 3.26 1.22–8.67 112 3.68 1.40–9.67 Infant birthweight, per 100 g 15,473 1.09 1.06–1.13 13,358 1.08 1.05–1.12 Modifiable variables Maternal birth position supine/sitting 11,121 Reference 10,287 Reference side bearing 2,105 0.53 0.29–0.95 1,936 0.73 0.40–1.32 kneeling/all fours 974 0.66 0.31–1.42 898 0.91 0.42–1.95 squatting 34 5.45 1.40–21.2 30 8.78 2.29–33.5 standing 107 2.60 0.84–8.04 98 3.45 1.13–10.5 water 500 0.19 0.03–1.32 447 - - outside hospital, unplanned 159 1.75 0.56–5.44 109 4.38 1.40–13.7 Oxytocin 2,892 2.93 2.10–4.08 2,809 2.00 1.39–2.88 Pudendal analgesia 84 7.75 3.52–17.0 80 3.98 1.83–8.68 Vacuum extraction 617 5.38 3.57–8.10 576 3.29 2.09–5.17 Discussion In this relatively large cohort study, exploring risk factors for OASI, we analyzed prospectively collected data from almost 30,000 births in the past decade in our regional perinatal database. The results from the analyses of these data confirmed many of the already established risk factors for OASI; former OASI, increased birthweight, occiput posterior position, use of oxytocin, and instrumental vaginal delivery. However, the primary aim of the study was to further explore amniotomy as a novel modifiable risk factor for OASI as a consequence of our recent findings in a case-control study. 8 Yet, in this cohort study based, on contemporary data from our regional birth registry, we did not find amniotomy to be associated with risk of OASI. The incidence of OASI in the studied region (Innlandet County, Norway) has decreased over time. In 2006, it dropped from around 4% in the early 2000s to approximately 2% and has remained at this lower level since, which is in line with national trends. 7 This decline could perhaps also be one of the explanations for why our results differ from our previous study. However, a major limitation of our current data was that we could not determine whether amniotomy was performed to induce, or to augment, labor, as this information was often not recorded. But to our knowledge, this is the most extensive study exploring amniotomy as a possible risk factor for OASI. When examining other potential risk factors, two variables—infant birthweight and vacuum extraction—were found to increase the risk of OASI in both primiparous and parous women. Instrumental vaginal delivery has long been recognized as an important risk factor for OASI, 12 , 13 and our data supports this finding. However, we did not observe a statistically significant association between forceps-assisted delivery and the risk of OASI among parous women. This may be attributable to the relatively small number of parous women who underwent forceps-assisted deliveries in the present study, potentially resulting in insufficient statistical power to detect a meaningful association. In Norway, episiotomy is commonly performed during instrumental vaginal deliveries, particularly for primiparous women, in accordance with national guidelines from the Norwegian Association of Gynecologists. 11 Our findings suggest that episiotomy was associated with a reduction in risk of OASI for primiparous women. This is in accordance with findings in a systematic review and meta-analysis from 2022, where it was found that mediolateral episiotomy may reduce the incidence of OASI, particularly in primiparous women. 14 Our study has several strengths. First, it benefits from structured data collected through the digital medical record, which provides detailed, systematically recorded information on maternal health, pregnancy, delivery, and the early postpartum period. The data are entered by midwives and doctors throughout the pregnancy and are transferred directly, without modification, to a regional birth registry (IPD). Second, the study includes a large, unselected population-based birth cohort, which enhances the generalizability of the results. Third, most variables have minimal missing data, which leads to more accurate estimates of relationships between variables, increases the statistical power of tests, and makes it easier to detect meaningful differences in the data, ultimately enhancing the reliability of the results. However, there are also some weaknesses. One limitation is the already mentioned unclear indication for amniotomy in our dataset, as we could not distinguish whether the procedure was performed for induction or augmentation due to the lack of specific documentation. Additionally, there is the potential for undiagnosed OASI, as some women may experience this injury without it being recognized at birth, with the diagnosis often only occurring later due to persistent symptoms like anal incontinence. Such cases are not included in our database, meaning women with delayed diagnoses of OASI are not represented in the study. Lastly, there were two variables—total duration of birth and maternal BMI—that had to be excluded from the analysis due to insufficient or unreliable data. Conclusions In this large cohort study based on structured data from a regional birth registry in the recent decade, we confirmed several known risk factors for OASI; increased birthweight, former OASI, occiput posterior position, use of oxytocin, and instrumental vaginal delivery, particularly vacuum extraction. Additionally, episiotomy was associated with a lower risk of OASI in primiparous women, although no such effect was observed in parous women. However, amniotomy was not confirmed as a factor increasing the risk for OASI. Future research is warranted to further refine our understanding risk factors for OASI and to develop further prevention strategies. Abbreviations GA gestational ager IHT Innlandet Hospital Trust IPD Innlandet Perinatal Database OASI obstetric anal sphincter injury RR relative risk Declarations Ethics statement The research project was approved by the Regional Ethical Committee for Health Research, region South-East D with the reference number 594756. The study was also approved by the Data Protection Officer at Innlandet Hospital Trust with the reference number 24334587. Data were extracted from our regional quality assurance data registry (IPD). Detailed information regarding maternal health, pregnancy, delivery, and the early postpartum period is recorded by midwives and doctors in a digital medical record and directly transferred to the IPD without modification structured quality assurance data. Refrainment from subsequent additional obtainment of written consent of included women in this retrospective study was approved by the Regional Ethical Committee for Health Research, region South-East D under the prerequisite that all data was anonymized and further that all data handling and analyses did not generate subgroups of less than five individuals. Consent for publication Not applicable Competing interests The authors declare that they have no competing interests. Funding The study was funded by Innlandet Hospital Trust research fund, grant number 150493. Author Contribution IEB: Data curation; Methodology; Formal analyses; Writing - original draft; Writing - review & editing.KSB: Conceptualization; Database administration; Data curation; Supervision; Methodology; Formal analyses; Writing - original draft; Writing - review & editing.MNH-A: Conceptualization; Data curation; Formal analyses; Funding acquisition; Investigation; Methodology; Project administration; Supervision; Resources; Software; Visualization; Writing - original draft; Writing - review & editing. Acknowledgements Not applicable Data Availability The data underlying this article cannot be shared publicly due to policy for internal quality assurance databases at Innlandet Hospital Trust. The data will be shared on reasonable request to the corresponding author. References Huebner M, Gramlich NK, Rothmund R, Nappi L, Abele H, Becker S. Fecal incontinence after obstetric anal sphincter injuries. Int J Gynaecol Obstet. 2013;121(1):74–7. 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Available online at: https://www.fhi.no/en/ch/medical-birth-registry-of-norway/ (Accessed October 12, 2024). Klokk R, Bakken KS, Markestad T, Holten-Andersen MN. Modifiable and non-modifiable risk factors for obstetric anal sphincter injury in a Norwegian Region: a case-control study. BMC Pregnancy Childbirth. 2022;22(1):277. Gaudernack LC, Frøslie KF, Michelsen TM, Voldner N, Lukasse M. De-medicalization of birth by reducing the use of oxytocin for augmentation among first-time mothers – a prospective intervention study. BMC Pregnancy Childbirth. 2018;18(1). Seijmonsbergen-Schermers AE, Zondag DC, Nieuwenhuijze M, et al. Regional variations in childbirth interventions and their correlations with adverse outcomes, birthplace and care provider: A nationwide explorative study. PLoS ONE. 2020;15(3):e0229488. Perinealskade og anal sfinkterskade ved fødsel. Available online at: https://metodebok.no/index.php?action=topic&item=qqnzK4BK (Accessed January 12, 2025). Dudding TC, Vaizey CJ, Kamm MA. Obstetric anal sphincter injury: incidence, risk factors, and management. Ann Surg. 2008;247(2):224–37. Ramm O, Woo VG, Hung Y-Y, Chen H-C, Ritterman Weintraub ML. Risk Factors for the Development of Obstetric Anal Sphincter Injuries in Modern Obstetric Practice. Obstet Gynecol. 2018;131(2):290–6. Okeahialam NA, Wong KW, Jha S, Sultan AH, Thakar R. Mediolateral/lateral episiotomy with operative vaginal delivery and the risk reduction of obstetric anal sphincter injury (OASI): A systematic review and meta-analysis. Int Urogynecol J. 2022;33(6):1393–405. Additional Declarations No competing interests reported. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-7299238","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":505465511,"identity":"25bf34ce-d50f-48d4-9d82-d2a1bf92f877","order_by":0,"name":"Ida Elisabeth Bang","email":"","orcid":"","institution":"Women’s Clinic, Innlandet Hospital Trust","correspondingAuthor":false,"prefix":"","firstName":"Ida","middleName":"Elisabeth","lastName":"Bang","suffix":""},{"id":505465512,"identity":"d8f61bf1-6b98-4ab4-af90-390718d74056","order_by":1,"name":"Kjersti Sletten Bakken","email":"","orcid":"","institution":"Women’s Clinic, Innlandet Hospital Trust","correspondingAuthor":false,"prefix":"","firstName":"Kjersti","middleName":"Sletten","lastName":"Bakken","suffix":""},{"id":505465513,"identity":"76564d60-da12-465e-80b5-dce6103e79dd","order_by":2,"name":"Mads Nikolaj Holten-Andersen","email":"data:image/png;base64,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","orcid":"","institution":"Innlandet Hospital Trust, Lillehammer Hospital","correspondingAuthor":true,"prefix":"","firstName":"Mads","middleName":"Nikolaj","lastName":"Holten-Andersen","suffix":""}],"badges":[],"createdAt":"2025-08-05 10:08:11","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-7299238/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7299238/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1186/s12884-025-08422-1","type":"published","date":"2025-12-25T15:58:01+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":89920606,"identity":"4a10a930-b27f-4655-8d66-5b97686cc946","added_by":"auto","created_at":"2025-08-26 12:39:16","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":81827,"visible":true,"origin":"","legend":"\u003cp\u003eFlow chart of data included in the study.\u003c/p\u003e","description":"","filename":"Figure1.png","url":"https://assets-eu.researchsquare.com/files/rs-7299238/v1/d33497f5b45a801ff9756abb.png"},{"id":89920609,"identity":"7190062e-b5e2-4d44-9afa-af9e770b8583","added_by":"auto","created_at":"2025-08-26 12:39:17","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":22222,"visible":true,"origin":"","legend":"\u003cp\u003eIncidence of OASI in Innlandet Hospital Trust\u003c/p\u003e","description":"","filename":"Figure2.png","url":"https://assets-eu.researchsquare.com/files/rs-7299238/v1/d5730f34476ed18432c141b0.png"},{"id":99172859,"identity":"04969498-fa4a-4dfc-b83c-35ead39b279c","added_by":"auto","created_at":"2025-12-29 16:11:39","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1029501,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7299238/v1/2af79c09-35c2-41c8-be8e-2e880cbb6772.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Risk factors for obstetric anal sphincter injury in a Norwegian Region: a cohort study","fulltext":[{"header":"Introduction","content":"\u003cp\u003eObstetric anal sphincter injury (OASI) is the term used to describe the unintentional tearing of the anal sphincter and/or the rectal mucosa at delivery. It is a feared complication of vaginal delivery because it carries a high risk of morbidity for affected women, with both Norwegian and international studies showing 40\u0026ndash;50% suffering from some degree of persisting anal incontinence despite adequate suturing.\u003csup\u003e\u003cspan additionalcitationids=\"CR2\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e Internationally, OASI has an incidence of about 3\u0026ndash;7% in women giving vaginal birth.\u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e In Norway, there was an increase in the reported number of OASI from less than 1% in the late 1960s to 4,3% in 2004.\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e This caused the Norwegian Board of Health to commission an interventional study, with promising results leading to the implementation of a national prevention program aiming to reduce the incidence of OASI.\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e A key feature of this program was manual support of the perineum. In line with the results of the interventional study, a significant decrease in OASI was seen after the intervention, with rates dropping as low as 1\u0026ndash;2%.\u003csup\u003e5, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e Rates of OASI have remained low in Norway and in 2022, OASI occurred in 1,8% of all vaginal births in Innlandet Hospital Trust, and in 1,6% of all vaginal births in Norway.\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\u003cp\u003ePrimiparity, high maternal age, instrumental vaginal delivery, augmentation with oxytocin, and a large fetus are some of the well-established risk factors.\u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e In addition, we recently demonstrated that the risk of OASI was significantly higher in women where amniotomy was performed, regardless of parity.\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e This could mean that amniotomy may be a hitherto overlooked significant independent modifiable risk factor for OASI regardless of parity. Amniotomy is a readily used procedure in the management of vaginal birth to shorten the length of labor by speeding up uterine contractions. However, the use of amniotomy in birth management is debated and most likely varies greatly both between as well as within institutions.\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\u003cp\u003eIn an updated systematic search of published studies, we have not found any other studies investigating amniotomy as a risk factor for OASI. The aim of this study is therefore to explore if amniotomy can be verified as a risk factor for OASI, in women giving vaginal birth in singleton pregnancies after 30 weeks of gestation in the Innlandet Hospital Trust (IHT), using a cohort study design including contemporary data from our regional birth registry.\u003c/p\u003e"},{"header":"Material and methods","content":"\u003cp\u003e\u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003eStudy design and population\u003c/span\u003e\u003c/p\u003e\u003cp\u003eWe conducted a retrospective cohort study using data from the Innlandet Perinatal Database (IPD). At IHT, detailed information regarding maternal health, pregnancy, delivery, and the early postpartum period is recorded in a digital medical record. Midwives and doctors input data throughout the pregnancy. This structured data is directly transferred to the IPD without modification. Data extraction from IPD for this study was performed retrospectively in September 2023 (see Table\u0026nbsp;1).\u003c/p\u003e\u003cp\u003eInnlandet County, located in eastern Norway, has a population of 371,385 (as of January 1, 2020) with most births occurring within IHT. In the 10-year study period from 2012\u0026ndash;2022, 30,445 births were registered in IPD.\u003c/p\u003e\u003cp\u003eThe study population includes all women who had a singleton vaginal birth after 30 weeks of gestation at Innlandet Hospital Trust from 2012 to 2022. This includes the hospitals in Lillehammer, Gj\u0026oslash;vik, Elverum and Tynset for the whole study period, and the hospital in Kongsvinger from 2012 to 2018 (before its transfer to the neighboring Hospital Trust from 2019). Lillehammer Hospital has the biggest obstetric department, followed by Elverum and Gj\u0026oslash;vik, while Tynset operates a small midwifery-led delivery ward with approximately 50 births per year.\u003c/p\u003e\u003cp\u003e\u003cstrong\u003eDefinitions\u003c/strong\u003e\u003cp\u003eThe outcome variable OASI was defined according to the ICD-10 classification as perineal tears grade 3, involving the external (3A and B) and internal (3C) anal sphincter (O70.2) or grade 4, where the anal or rectal mucosa is involved in addition to the external and internal sphincter (O70.3).\u003csup\u003e\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u003c/sup\u003e The diagnosis was made at the time of delivery, confirmed by the on-call specialist in obstetrics and gynecology.\u003c/p\u003e\u003c/p\u003e\u003cp\u003eWe included the following independent non-modifiable and modifiable variables that could affect the birth process. Non-modifiable variables included birthweight (grams and quartiles), head circumference (cm), gestational age (GA) (weeks and categories), maternal age (years and categories), duration of the second stage of labor (\u0026lt;\u0026thinsp;60 min or \u0026gt;\u0026thinsp;60 min), fetal presentation (occiput anterior, occiput posterior, breech, other vertex presentations), and former OASI. Modifiable variables included induction of labor (yes/no), amniotomy (yes/no), use of oxytocin (yes/no), augmentation (yes/no), induction (yes/no), episiotomy (yes/no), maternal birth position (supine/sitting, side bearing, kneeling/all fours, squatting, standing, water, outside of hospital - unplanned), instrumental delivery by vacuum extraction (yes/no) or forceps (yes/no), and finally modes of pain relief; epidural (yes/no), nitrous oxide inhalation (yes/no), or pudendal nerve block (yes/no).\u003c/p\u003e\u003cp\u003eFor most of the population, GA was estimated according to routine ultrasonography at 18\u0026ndash;20 weeks of gestation. At birth, weight and head circumference were measured according to protocol. Induction of labor included transcervical Foley catheter, vaginal or oral prostaglandin, amniotomy or/and intravenous oxytocin. Amniotomy was also performed in births where continuous surveillance of the fetus with an internal electrode was indicated or in the case of labor dystocia, often in combination with augmentation with intravenous oxytocin.\u003c/p\u003e\u003cp\u003eInstrumental vaginal delivery, vacuum extraction or forceps, was performed in the case of signs of fetal asphyxia or dystocia of the second stage of labor. Episiotomy was always mediolateral. Indications for episiotomy were instrumental vaginal delivery, breech delivery, former OASI, suspected fetal asphyxia or if risk of perineal rupture during delivery was imminent, at the discretion of the clinician.\u003c/p\u003e\u003cp\u003eThe duration of the second stage of labor was routinely recorded immediately after birth.\u003c/p\u003e\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003eStatistical analysis\u003c/h2\u003e\u003cp\u003eFor descriptive statistical analyses, continuous variables were described by means and standard deviations. Categorical variables were described by frequencies and proportions.\u003c/p\u003e\u003cp\u003eSeparate analyses were performed for the primiparous and parous (one or more previous deliveries) women. The data were quality-assured and to some degree expanded by going through relevant delivery protocols. A p-value\u0026thinsp;\u0026lt;\u0026thinsp;0,05 was chosen as the level of significance.\u003c/p\u003e\u003cp\u003eThe included modifiable and non-modifiable independent variables were assessed with univariate regression analyses to explore the associations between the variables and risk of OASI.\u003c/p\u003e\u003cp\u003eWe then developed a risk-factor model for OASI using multivariate logistic regression with a stepwise procedure, implemented through generalized linear models in Stata. We specified the model with a binomial family and a logit link to estimate risk ratios. In each step of the procedure, we used backward elimination; the variable with the least statistical significance was removed until all remaining exposure variables showed a significant association with OASI. STATA 18.0 software (StataCorp, College Station, TX) was used for all the analyses.\u003c/p\u003e\u003c/div\u003e"},{"header":"Results","content":"\u003cp\u003eIn the decade from 2012 to 2022 there were 33,572 births registered in IHT. In the analyses, we included all singleton vaginal births in the period from 2012 to 2022 and excluded births by caesarean section, twin births, and births before gestational week 30. This resulted in a total number of 26,349 included births (Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e).\u003c/p\u003e\n\u003cp\u003eThe yearly total incidence of OASI in IHT has been around 2% for the duration of the defined period of our study. The ratio of OASI in women with spontaneous delivery is lower than in the group with instrumental vaginal delivery (vacuum and forceps). (Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e). The annual number of women giving birth during the study period has decreased gradually from 2,934 births in 2012 to 2,482 births in 2022, a 18% decline in the total number of births.\u003c/p\u003e\n\u003cp\u003eTable\u0026nbsp;1a and 1b describes the modifiable and non-modifiable factors for primiparous and parous women with and without OASI. There were 10,544 primiparous and 15,343 parous women that met the inclusion criteria, the incidence of OASI was 2.9% in the primiparous group and 1.0% in the parous group.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003cstrong\u003eTable 1a\u0026nbsp;\u003c/strong\u003eStudy characteristics of included primiparous women\u003c/p\u003e\n\u003ctable border=\"0\" cellspacing=\"0\" cellpadding=\"0\" width=\"605\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e\u003cstrong\u003ePrimiparous women\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003eno OASI\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003eOASI\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003emean (SD) or n (%)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003emean (SD) or n (%)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003ep-value\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003eN\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e10544 (97.1)\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e314 (2.9)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eNon-modifiable variables\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\u003cbr\u003e\u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\u003cbr\u003e\u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003eMaternal age (years)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e27.8 (4.7)\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e29.3 (4.8)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e\u0026lt; 0.001\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003eMaternal age category\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\u003cbr\u003e\u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\u003cbr\u003e\u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e\u0026lt; 0.001\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cem\u003e\u0026lt;25 years\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e2573 (24.4)\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e47 (15.0)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cem\u003e25-29 years\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e4455 (42.3)\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e134 (42.7)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cem\u003e30-34 years\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e2618 (24.8)\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e85 (27.1)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cem\u003e\u0026gt;35 years\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e898 (8.5)\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e48 (15.3)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003eGestational age (weeks)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e39.6 (1.6)\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e40.0 (1.3)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e\u0026lt; 0.001\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003eGestational age categories\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\u003cbr\u003e\u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\u003cbr\u003e\u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e0.004\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cem\u003eat term (37-42 weeks)\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e9335 (88.5)\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e278 (88.5)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cem\u003epreterm (\u0026lt;37 weeks)\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e424 (4.0)\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e3 (1.0)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cem\u003epost term (\u0026gt;42 weeks)\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e785 (7.4)\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e33 (10.5)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003eFetal presentation\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\u003cbr\u003e\u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\u003cbr\u003e\u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e\u0026lt; 0.001\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cem\u003eocciput anterior\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e9662 (91.7)\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e262 (83.4)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cem\u003eocciput posterior\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e727 (6.9)\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e48 (15.3)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cem\u003ebreech\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e83 (0.8)\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e1 (0.3)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cem\u003eother vertex presentations\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e60 (0.6)\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e3 (1.0)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003eDuration of 2nd stage\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\u003cbr\u003e\u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\u003cbr\u003e\u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e0.04\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u0026le;\u003cem\u003e60 min\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e8291 (80.4)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e236 (75.6)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cem\u003e\u0026gt;60 min\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e2022 (19.6)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e76 (24.4)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003eWeight (g)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e3442.1 (472.2)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e3639.8 (451.4)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e\u0026lt; 0.001\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003eBirthweight quartiles\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\u003cbr\u003e\u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\u003cbr\u003e\u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e\u0026lt; 0.001\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cem\u003e\u0026lt; 3240 g\u0026nbsp;\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e3397 (32.3)\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e50 (15.9)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cem\u003e3240 - 3549 g\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e2829 (26.9)\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e86 (27.4)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cem\u003e3560 g - 3865 g\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e2446 (23.2)\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e84 (26.8)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cem\u003e\u0026gt; 3866 g\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e1854 (17.6)\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e94 (29.9)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003eHead circumference (cm)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e34.8 (1.5)\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e35.3 (1.5)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e\u0026lt; 0.001\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eModifiable variables\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\u003cbr\u003e\u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\u003cbr\u003e\u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003eInduction of labor\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e2433 (23.1)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e\u0026nbsp;91 (29.0)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e0.02\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003eAmniotomy\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e4040 (39.1)\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e120 (38.2)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e0.7\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003eOxytocin (augmentation or induction)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e5455 (54.4)\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e194 (63.8)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e0.001\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003eMaternal birth position\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\u003cbr\u003e\u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\u003cbr\u003e\u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e0.2\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cem\u003esupine/sitting\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e8855 (85.9)\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e269 (85.9)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cem\u003eside bearing\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e885 (8.6)\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e26 (8.3)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cem\u003ekneeling/all fours\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e364 (3.5)\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e9 (2.9)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cem\u003esquatting\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e26 (0.3)\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e0 (0.0)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cem\u003estanding\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e36 (0.3)\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e2 (0.6)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cem\u003ewater\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e131 (1.3)\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e5 (1.6)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cem\u003eoutside hospital, unplanned\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e11 (0.1)\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e2 (0.6)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003ePain relief\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\u003cbr\u003e\u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\u003cbr\u003e\u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003eEpidural\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e4814 (45.7)\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e132 (42.0)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e0.2\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003eNitrous oxide inhalation\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e7768 (73.7)\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e235 (74.8)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e0.6\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003ePudendal\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e177 (1.7)\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e16 (5.1)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e\u0026lt; 0.001\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003eInstrumental delivery\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\u003cbr\u003e\u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\u003cbr\u003e\u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003eVacuum extraction\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e1866 (17.7)\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e101 (32.2)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e\u0026lt; 0.001\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003eForceps\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e327 (3.1)\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e35 (11.1)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e\u0026lt; 0.001\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003eEpisiotomy\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e3944 (39.3)\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e133 (42.4)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 76px;\"\u003e\n \u003cp\u003e0.3\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003e\u0026nbsp;\u003cstrong\u003eTable 1b\u0026nbsp;\u003c/strong\u003eStudy characteristics of included parous women\u003c/p\u003e\n\u003ctable border=\"0\" cellspacing=\"0\" cellpadding=\"0\" width=\"605\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eParous women\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003eno OASI\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003eOASI\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003emean (SD) or n (%)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003emean (SD) or n (%)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003ep-value\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003eN\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e15,343 (99.0) \u0026nbsp; \u0026nbsp; \u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e148 (1.0)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eNon-modifiable variables\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003eMaternal age (years)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e31.4 (4.6) \u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e31.5 (4.3)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e0.6\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003eMaternal age category\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e0.96\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cem\u003e\u0026lt;25 years\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e964 (6.3) \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e9 (6.1)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cem\u003e25-29 years\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e4,589 (29.9) \u0026nbsp; \u0026nbsp; \u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e43 (29.1)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cem\u003e30-34 years\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e5,995 (39.1) \u0026nbsp; \u0026nbsp; \u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e61 (41.2)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cem\u003e\u0026gt;35 years\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e3,795 (24.7) \u0026nbsp; \u0026nbsp; \u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e35 (23.6)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003eFormer OASI\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e127 (0.8) \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e4 (2.7)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e0.01\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003eGestational age (weeks)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e39.6 (1.4) \u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e39.8 (1.3)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e0.1\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003eGestational age categories\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e0.004\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cem\u003eat term (37-42 weeks)\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e14,215 (92.6) \u0026nbsp; \u0026nbsp;\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e132 (89.2)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cem\u003epreterm (\u0026lt;37 weeks)\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e399 (2.6) \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e1 (0.7)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cem\u003epost term (\u0026gt;42 weeks)\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e729 (4.8) \u0026nbsp; \u0026nbsp; \u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e15 (10.1)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003eFetal presentation\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e0.4\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cem\u003eocciput anterior\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e14,116 (92.1)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e133 (89.9)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cem\u003eocciput posterior\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e1047 (6.8) \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e14 (9.5)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cem\u003ebreech\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e102 (0.7) \u0026nbsp; \u0026nbsp; \u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e0 (0.0)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cem\u003eother vertex presentations\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e60 (0.4) \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e1 (0.7)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003eDuration of 2nd stage\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e\u0026lt; 0.001\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u0026le;\u003cem\u003e60 min\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e14,478 (97.7)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e134 (90.5)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cem\u003e\u0026gt;60 min\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e348 (2.3)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e14 (9.5)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003eWeight (g)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e3,621.9 (477.5)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e3,825.2 (451.6)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e\u0026lt; 0.001\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003eBirthweight quartiles\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e\u0026lt; 0.001\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cem\u003e\u0026lt; 3240 g\u0026nbsp;\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e3,087 (20.1) \u0026nbsp; \u0026nbsp; \u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e16 (10.8)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cem\u003e3240 - 3549 g\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e3,592 (23.4) \u0026nbsp; \u0026nbsp; \u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e25 (16.9)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cem\u003e3560 g - 3865 g\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e4,081 (26.6) \u0026nbsp; \u0026nbsp; \u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e36 (24.3)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cem\u003e\u0026gt; 3866 g\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e4,565 (29.8) \u0026nbsp; \u0026nbsp; \u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e71 (48.0)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003eHead circumference (cm)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e35.2 (1.4) \u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e35.6 (1.4)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e0.001\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eModifiable variables\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003eInduction of labor\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e3,270 (21.3) \u0026nbsp; \u0026nbsp; \u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e44 (29.7)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e0.01\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003eAmniotomy\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e5,595 (37.1) \u0026nbsp; \u0026nbsp; \u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e57 (38.5)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e0.7\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003eOxytocin (augmentation or induction)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e2,832 (20.2) \u0026nbsp; \u0026nbsp; \u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e60 (42.9)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e\u0026lt; 0.001\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003eMaternal birth position\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e0.001\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cem\u003esupine/sitting\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e11,001 (74.1) \u0026nbsp; \u0026nbsp;\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e120 (81.1)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cem\u003eside bearing\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e2,093 (14.1) \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e12 (8.1)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cem\u003ekneeling/all fours\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e967 (6.5) \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e7 (4.7)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cem\u003esquatting\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e32 (0.2) \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e2 (1.4)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cem\u003estanding\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e104 (0.7) \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e3 (2.0)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cem\u003ewater\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e499 (3.4) \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e1 (0.7)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003e\u003cem\u003eoutside hospital, unplanned\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e156 (1.1) \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e3 (2.0)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003ePain relief\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003eEpidural\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e2,891 (18.8) \u0026nbsp; \u0026nbsp; \u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e52 (35.1)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e\u0026lt; 0.001\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003eNitrous oxide inhalation\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e9,311 (60.7) \u0026nbsp; \u0026nbsp; \u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e88 (59.5)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e0.8\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003ePudendal\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e78 (0.5) \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e6 (4.1)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e\u0026lt; 0.001\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003eInstrumental delivery\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003eVacuum extraction\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e590 (3.8) \u0026nbsp; \u0026nbsp; \u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e27 (18.2)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e\u0026lt; 0.001\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003eForceps\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e119 (0.8) \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e4 (2.7)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e0.009\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" style=\"width: 255px;\"\u003e\n \u003cp\u003eEpisiotomy\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 142px;\"\u003e\n \u003cp\u003e1,361 (9.6) \u0026nbsp; \u0026nbsp; \u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 132px;\"\u003e\n \u003cp\u003e32 (21.6)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" style=\"width: 75px;\"\u003e\n \u003cp\u003e\u0026lt; 0.001\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFindings for primiparous women (Table\u0026nbsp;1a)\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWomen with OASI were, on average, 1.5 years older and more likely to have post-term pregnancies. Their infants had higher weights and larger head circumferences. OASI was more frequent when the second stage of labor exceeded 60 minutes, with occiput posterior positioning, and in cases of labor induction, oxytocin use and instrumental vaginal delivery. Epidural use was higher in women without OASI.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFindings for parous women (Table\u0026nbsp;1b)\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eSimilar to primiparous women, parous women with OASI had infants with higher mean birthweight, larger mean head circumferences, a higher proportion with more than 60 min duration of second-stage of labor, a higher proportion of infants with occiput posterior presentation, and a higher proportion of post-term pregnancies. Further, among women with OASI, a higher proportion had labor induction, use of oxytocin, instrumental vaginal deliveries, epidural, and episiotomies.\u003c/p\u003e\n\u003cp\u003eThe multivariate logistic regression analysis revealed that among primiparous women, an increase in infant birthweight, per 100 g, was associated with an increased risk of OASI with a relative risk (RR) of 1.09 (95% CI 1.07\u0026ndash;1.12 [Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e]). Additionally, increasing maternal age was associated with an increase in risk of OASI in this group, one year increase in age gives an increase in RR of 1.06 (95% CI 1.03\u0026ndash;1.08). Among the modifiable variables, having an instrumental delivery was associated with an increase in risk of OASI, for both vacuum (RR 2.72, 95% CI 2.06\u0026ndash;3.62) and forceps delivery (RR 5.67, 95% CI 3.96-9.00) when compared to spontaneous vaginal delivery. Furthermore, having an infant in occiput posterior presentation was associated with an increased risk of OASI when compared to an infant with occiput anterior presentation (RR 1.89, 95% CI 1.40\u0026ndash;2.54). Having an episiotomy or an epidural analgesia was associated with a decreased risk of OASI among primiparous with RR of 0.53 (95% CI 0.40\u0026ndash;0.70) and RR of 0.69 (95% CI 0.55\u0026ndash;0.86), respectively.\u003c/p\u003e\n\u003cp\u003e\u003cbr\u003e\u003c/p\u003e\n\u003cdiv class=\"gridtable\"\u003e\n \u003ctable id=\"Tab1\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eMultivariate logistic regression analysis for OASI among primiparous women, N\u0026thinsp;=\u0026thinsp;10,310\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\u0026nbsp;\u003c/th\u003e\n \u003cth align=\"left\" colspan=\"3\"\u003e\n \u003cp\u003eCrude\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colspan=\"3\"\u003e\n \u003cp\u003eAdjusted\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eVariable\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003en\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eRR\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003e95% CI\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003en\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eRR\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003e95% CI\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eNon-modifiable variables\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\u0026nbsp;\u003c/th\u003e\n \u003cth align=\"left\"\u003e\u0026nbsp;\u003c/th\u003e\n \u003cth align=\"left\"\u003e\u0026nbsp;\u003c/th\u003e\n \u003cth align=\"left\"\u003e\u0026nbsp;\u003c/th\u003e\n \u003cth align=\"left\"\u003e\u0026nbsp;\u003c/th\u003e\n \u003cth align=\"left\"\u003e\u0026nbsp;\u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eMaternal age, years\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e10,858\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.06\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1.04\u0026ndash;1.09\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e10,310\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.06\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1.03\u0026ndash;1.08\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eFetal presentation\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cem\u003eocciput anterior\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e9,924\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eReference\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e9,440\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eReference\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cem\u003eocciput posterior\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e775\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.35\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1.74\u0026ndash;3.16\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e727\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.89\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1.40\u0026ndash;2.54\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cem\u003ebreech\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e84\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.45\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.06\u0026ndash;3.18\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e83\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.49\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.07\u0026ndash;3.40\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cem\u003eother vertex presentations\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e63\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.80\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.59\u0026ndash;5.48\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e60\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.61\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.53\u0026ndash;4.84\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eInfant birthweight, per 100 g\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e10,840\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.09\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1.07\u0026ndash;1.12\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e10,310\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.09\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1.07\u0026ndash;1.12\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cstrong\u003eModifiable variables\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eEpidural\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e4,946\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.87\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.70\u0026ndash;1.08\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e4,720\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.69\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.55\u0026ndash;0.86\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eVacuum extraction\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1,967\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.14\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1.70\u0026ndash;2.70\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1,961\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.72\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e2.06\u0026ndash;3.61\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eForceps\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e362\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3.64\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e2.60\u0026ndash;5.09\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e362\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e5.97\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e3.96\u0026ndash;9.00\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eEpisiotomy\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e4,077\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.13\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.91\u0026ndash;1.41\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e4,063\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.53\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.40\u0026ndash;0.70\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n\u003c/div\u003e\n\u003cp\u003e\u003cbr\u003e\u003c/p\u003e\n\u003cp\u003eFor parous women, former OASI and an increase in infant birthweight were associated with an increased risk of OASI with RR of 3.68 (95% CI 1.40\u0026ndash;9.67) and 1.08 (95% CI 1.05\u0026ndash;1.12), respectively (Table\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e). A 100 g increase in infant birthweight was estimated to increase the risk of OASI by 8%. Among the modifiable variables, having a vacuum extraction was associated with an increased risk of OASI when compared to spontaneous vaginal delivery with a RR of 3.29 (95% CI 2.09\u0026ndash;5.17). If the mother had a squatting (RR 8.78, 95% CI 2.29\u0026ndash;33.5) or standing birth position (RR 3.45, 95% CI 1.13\u0026ndash;10.5) when compared to a supine position, was also associated with an increase in risk of OASI. Having an unplanned out-of-hospital birth or having received oxytocin infusion also increased the risk of OASI with RR of 4.38 (95% CI 1.40\u0026ndash;13.7) and 2.00 (95% CI 1.39\u0026ndash;2.88), respectively.\u003c/p\u003e\n\u003cp\u003e\u003cbr\u003e\u003c/p\u003e\n\u003cdiv class=\"gridtable\"\u003e\n \u003ctable id=\"Tab2\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eMultivariate logistic regression analysis for OASI among multiparous women. N\u0026thinsp;=\u0026thinsp;13,358\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\u0026nbsp;\u003c/th\u003e\n \u003cth align=\"left\"\u003e\u0026nbsp;\u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eCrude\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\u0026nbsp;\u003c/th\u003e\n \u003cth align=\"left\"\u003e\u0026nbsp;\u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eAdjusted\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\u0026nbsp;\u003c/th\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eVariable\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003en\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eRR\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003e95% CI\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003en\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eRR\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003e95% CI\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eNon-modifiable variables\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\u0026nbsp;\u003c/th\u003e\n \u003cth align=\"left\"\u003e\u0026nbsp;\u003c/th\u003e\n \u003cth align=\"left\"\u003e\u0026nbsp;\u003c/th\u003e\n \u003cth align=\"left\"\u003e\u0026nbsp;\u003c/th\u003e\n \u003cth align=\"left\"\u003e\u0026nbsp;\u003c/th\u003e\n \u003cth align=\"left\"\u003e\u0026nbsp;\u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eFormer OASI\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e131\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3.26\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1.22\u0026ndash;8.67\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e112\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3.68\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.40\u0026ndash;9.67\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eInfant birthweight, per 100 g\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e15,473\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.09\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1.06\u0026ndash;1.13\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e13,358\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.08\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.05\u0026ndash;1.12\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cstrong\u003eModifiable variables\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eMaternal birth position\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cem\u003esupine/sitting\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e11,121\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eReference\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e10,287\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eReference\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cem\u003eside bearing\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e2,105\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.53\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.29\u0026ndash;0.95\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1,936\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.73\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.40\u0026ndash;1.32\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cem\u003ekneeling/all fours\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e974\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.66\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.31\u0026ndash;1.42\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e898\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.91\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.42\u0026ndash;1.95\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cem\u003esquatting\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e34\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e5.45\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1.40\u0026ndash;21.2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e30\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e8.78\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.29\u0026ndash;33.5\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cem\u003estanding\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e107\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.60\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.84\u0026ndash;8.04\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e98\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3.45\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.13\u0026ndash;10.5\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cem\u003ewater\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e500\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.19\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.03\u0026ndash;1.32\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e447\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cem\u003eoutside hospital, unplanned\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e159\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.75\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.56\u0026ndash;5.44\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e109\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4.38\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.40\u0026ndash;13.7\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eOxytocin\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e2,892\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.93\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e2.10\u0026ndash;4.08\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e2,809\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.00\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.39\u0026ndash;2.88\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003ePudendal analgesia\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e84\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e7.75\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e3.52\u0026ndash;17.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e80\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3.98\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.83\u0026ndash;8.68\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eVacuum extraction\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e617\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e5.38\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e3.57\u0026ndash;8.10\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e576\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3.29\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.09\u0026ndash;5.17\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n\u003c/div\u003e"},{"header":"Discussion","content":"\u003cp\u003eIn this relatively large cohort study, exploring risk factors for OASI, we analyzed prospectively collected data from almost 30,000 births in the past decade in our regional perinatal database. The results from the analyses of these data confirmed many of the already established risk factors for OASI; former OASI, increased birthweight, occiput posterior position, use of oxytocin, and instrumental vaginal delivery.\u003c/p\u003e\u003cp\u003eHowever, the primary aim of the study was to further explore amniotomy as a novel modifiable risk factor for OASI as a consequence of our recent findings in a case-control study.\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e Yet, in this cohort study based, on contemporary data from our regional birth registry, we did not find amniotomy to be associated with risk of OASI.\u003c/p\u003e\u003cp\u003eThe incidence of OASI in the studied region (Innlandet County, Norway) has decreased over time. In 2006, it dropped from around 4% in the early 2000s to approximately 2% and has remained at this lower level since, which is in line with national trends.\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e This decline could perhaps also be one of the explanations for why our results differ from our previous study. However, a major limitation of our current data was that we could not determine whether amniotomy was performed to induce, or to augment, labor, as this information was often not recorded. But to our knowledge, this is the most extensive study exploring amniotomy as a possible risk factor for OASI.\u003c/p\u003e\u003cp\u003eWhen examining other potential risk factors, two variables\u0026mdash;infant birthweight and vacuum extraction\u0026mdash;were found to increase the risk of OASI in both primiparous and parous women. Instrumental vaginal delivery has long been recognized as an important risk factor for OASI,\u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e and our data supports this finding. However, we did not observe a statistically significant association between forceps-assisted delivery and the risk of OASI among parous women. This may be attributable to the relatively small number of parous women who underwent forceps-assisted deliveries in the present study, potentially resulting in insufficient statistical power to detect a meaningful association.\u003c/p\u003e\u003cp\u003eIn Norway, episiotomy is commonly performed during instrumental vaginal deliveries, particularly for primiparous women, in accordance with national guidelines from the Norwegian Association of Gynecologists.\u003csup\u003e\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u003c/sup\u003e Our findings suggest that episiotomy was associated with a reduction in risk of OASI for primiparous women. This is in accordance with findings in a systematic review and meta-analysis from 2022, where it was found that mediolateral episiotomy may reduce the incidence of OASI, particularly in primiparous women.\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\u003cp\u003eOur study has several strengths. First, it benefits from structured data collected through the digital medical record, which provides detailed, systematically recorded information on maternal health, pregnancy, delivery, and the early postpartum period. The data are entered by midwives and doctors throughout the pregnancy and are transferred directly, without modification, to a regional birth registry (IPD). Second, the study includes a large, unselected population-based birth cohort, which enhances the generalizability of the results. Third, most variables have minimal missing data, which leads to more accurate estimates of relationships between variables, increases the statistical power of tests, and makes it easier to detect meaningful differences in the data, ultimately enhancing the reliability of the results.\u003c/p\u003e\u003cp\u003eHowever, there are also some weaknesses. One limitation is the already mentioned unclear indication for amniotomy in our dataset, as we could not distinguish whether the procedure was performed for induction or augmentation due to the lack of specific documentation. Additionally, there is the potential for undiagnosed OASI, as some women may experience this injury without it being recognized at birth, with the diagnosis often only occurring later due to persistent symptoms like anal incontinence. Such cases are not included in our database, meaning women with delayed diagnoses of OASI are not represented in the study. Lastly, there were two variables\u0026mdash;total duration of birth and maternal BMI\u0026mdash;that had to be excluded from the analysis due to insufficient or unreliable data.\u003c/p\u003e"},{"header":"Conclusions","content":"\u003cp\u003eIn this large cohort study based on structured data from a regional birth registry in the recent decade, we confirmed several known risk factors for OASI; increased birthweight, former OASI, occiput posterior position, use of oxytocin, and instrumental vaginal delivery, particularly vacuum extraction. Additionally, episiotomy was associated with a lower risk of OASI in primiparous women, although no such effect was observed in parous women. However, amniotomy was not confirmed as a factor increasing the risk for OASI. Future research is warranted to further refine our understanding risk factors for OASI and to develop further prevention strategies.\u003c/p\u003e"},{"header":"Abbreviations","content":"\u003cdiv class=\"DefinitionList\"\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003eGA\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003egestational ager\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003eIHT\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003eInnlandet Hospital Trust\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003eIPD\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003eInnlandet Perinatal Database\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003eOASI\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003eobstetric anal sphincter injury\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003eRR\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003erelative risk\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003c/div\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cb\u003eEthics statement\u003c/b\u003e\u003c/p\u003e\u003cp\u003eThe research project was approved by the Regional Ethical Committee for Health Research, region South-East D with the reference number 594756. The study was also approved by the Data Protection Officer at Innlandet Hospital Trust with the reference number 24334587. Data were extracted from our regional quality assurance data registry (IPD). Detailed information regarding maternal health, pregnancy, delivery, and the early postpartum period is recorded by midwives and doctors in a digital medical record and directly transferred to the IPD without modification structured quality assurance data. Refrainment from subsequent additional obtainment of written consent of included women in this retrospective study was approved by the Regional Ethical Committee for Health Research, region South-East D under the prerequisite that all data was anonymized and further that all data handling and analyses did not generate subgroups of less than five individuals.\u003c/p\u003e\u003cp\u003e\u003cstrong\u003eConsent for publication\u003c/strong\u003e\u003cp\u003eNot applicable\u003c/p\u003e\u003c/p\u003e\u003cp\u003e\u003ch2\u003eCompeting interests\u003c/h2\u003e\u003cp\u003eThe authors declare that they have no competing interests.\u003c/p\u003e\u003c/p\u003e\u003ch2\u003eFunding\u003c/h2\u003e\u003cp\u003eThe study was funded by Innlandet Hospital Trust research fund, grant number 150493.\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eIEB: Data curation; Methodology; Formal analyses; Writing - original draft; Writing - review \u0026amp; editing.KSB: Conceptualization; Database administration; Data curation; Supervision; Methodology; Formal analyses; Writing - original draft; Writing - review \u0026amp; editing.MNH-A: Conceptualization; Data curation; Formal analyses; Funding acquisition; Investigation; Methodology; Project administration; Supervision; Resources; Software; Visualization; Writing - original draft; Writing - review \u0026amp; editing.\u003c/p\u003e\u003ch2\u003eAcknowledgements\u003c/h2\u003e\u003cp\u003eNot applicable\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eThe data underlying this article cannot be shared publicly due to policy for internal quality assurance databases at Innlandet Hospital Trust. The data will be shared on reasonable request to the corresponding author.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eHuebner M, Gramlich NK, Rothmund R, Nappi L, Abele H, Becker S. Fecal incontinence after obstetric anal sphincter injuries. Int J Gynaecol Obstet. 2013;121(1):74\u0026ndash;7.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eLaine K, Skjeldestad FE, Sandvik L, Staff AC. Prevalence and Risk Indicators for Anal Incontinence among Pregnant Women. Int Sch Res Notices. 2013;2013(1):947572.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eSideris M, McCaughey T, Hanrahan JG, et al. Risk of obstetric anal sphincter injuries (OASIS) and anal incontinence: A meta-analysis. Eur J Obstet Gynecol Reprod Biol. 2020;252:303\u0026ndash;12.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003ePergialiotis V, Bellos I, Fanaki M, Vrachnis N, Doumouchtsis SK. Risk factors for severe perineal trauma during childbirth: An updated meta-analysis. Eur J Obstet Gynecol Reprod Biol. 2020;247:94\u0026ndash;100.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eHals E, \u0026Oslash;ian P, Pirhonen T, et al. A Multicenter Interventional Program to Reduce the Incidence of Anal Sphincter Tears. Obstet Gynecol. 2010;116(4):901\u0026ndash;8.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eLaine K, Skjeldestad FE, Sandvik L, Staff AC. Incidence of obstetric anal sphincter injuries after training to protect the perineum: cohort study. BMJ Open. 2012;2(5).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eMedical Birth Registry of Norway. Available online at: \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://www.fhi.no/en/ch/medical-birth-registry-of-norway/\u003c/span\u003e\u003cspan address=\"https://www.fhi.no/en/ch/medical-birth-registry-of-norway/\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (Accessed October 12, 2024).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eKlokk R, Bakken KS, Markestad T, Holten-Andersen MN. Modifiable and non-modifiable risk factors for obstetric anal sphincter injury in a Norwegian Region: a case-control study. BMC Pregnancy Childbirth. 2022;22(1):277.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eGaudernack LC, Fr\u0026oslash;slie KF, Michelsen TM, Voldner N, Lukasse M. De-medicalization of birth by reducing the use of oxytocin for augmentation among first-time mothers \u0026ndash; a prospective intervention study. BMC Pregnancy Childbirth. 2018;18(1).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eSeijmonsbergen-Schermers AE, Zondag DC, Nieuwenhuijze M, et al. Regional variations in childbirth interventions and their correlations with adverse outcomes, birthplace and care provider: A nationwide explorative study. PLoS ONE. 2020;15(3):e0229488.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003ePerinealskade og anal sfinkterskade ved f\u0026oslash;dsel. Available online at: \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://metodebok.no/index.php?action=topic\u0026amp;item=qqnzK4BK\u003c/span\u003e\u003cspan address=\"https://metodebok.no/index.php?action=topic\u0026amp;item=qqnzK4BK\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (Accessed January 12, 2025).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eDudding TC, Vaizey CJ, Kamm MA. Obstetric anal sphincter injury: incidence, risk factors, and management. Ann Surg. 2008;247(2):224\u0026ndash;37.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eRamm O, Woo VG, Hung Y-Y, Chen H-C, Ritterman Weintraub ML. Risk Factors for the Development of Obstetric Anal Sphincter Injuries in Modern Obstetric Practice. Obstet Gynecol. 2018;131(2):290\u0026ndash;6.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eOkeahialam NA, Wong KW, Jha S, Sultan AH, Thakar R. Mediolateral/lateral episiotomy with operative vaginal delivery and the risk reduction of obstetric anal sphincter injury (OASI): A systematic review and meta-analysis. Int Urogynecol J. 2022;33(6):1393\u0026ndash;405.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"bmc-pregnancy-and-childbirth","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"prch","sideBox":"Learn more about [BMC Pregnancy and Childbirth](http://bmcpregnancychildbirth.biomedcentral.com/)","snPcode":"","submissionUrl":"https://www.editorialmanager.com/prch/default.aspx","title":"BMC Pregnancy and Childbirth","twitterHandle":"@BMC_series","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"em","reportingPortfolio":"BMC Series","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"OASI, amniotomy, instrumental delivery, risk factor, parity","lastPublishedDoi":"10.21203/rs.3.rs-7299238/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7299238/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003ch2\u003eBackground\u003c/h2\u003e\u003cp\u003eObstetric anal sphincter injury refers to the unintended tearing of the anal sphincter and/or rectal mucosa during vaginal delivery, a complication associated with significant morbidity. Well-known risk factors for obstetric anal sphincter injury include primiparity, high maternal age, instrumental delivery, oxytocin augmentation, and large fetal size. Recently, we identified amniotomy as a potential novel independent risk factor, regardless of parity. This study aims to further explore whether amniotomy increases the risk of obstetric anal sphincter injury in singleton vaginal births after 30 weeks of gestation at Innlandet Hospital Trust, using a cohort study design.\u003c/p\u003e\u003ch2\u003eMethods\u003c/h2\u003e\u003cp\u003eRetrospective cohort study using data from the Innlandet Perinatal Database, including all women who had singleton vaginal births after 30 weeks of gestation at Innlandet Hospital Trust from 2012 to 2022. We used multivariate logistic regression models to develop a risk-factor model for obstetric anal sphincter injury employing backward elimination with stratification for parity.\u003c/p\u003e\u003ch2\u003eResults\u003c/h2\u003e\u003cp\u003eThe annual incidence of obstetric anal sphincter injury at the hospitals was stable around 2% in the study period and a total of 26349 births were included. Amniotomy was not found to be a risk factor for obstetric anal sphincter injury in this contemporary cohort. Overall, instrumental delivery was identified as an independent modifiable risk factor for obstetric anal sphincter injury: vacuum extraction presenting relative risks of 2.72 and 3.29 in primiparous and parous women and forceps 5.97 in primiparous women. Episiotomy and epidural analgesia were associated with reduced risks of obstetric anal sphincter injury, but only in primiparous women (relative risks: 0.53 and 0.69).\u003c/p\u003e\u003ch2\u003eConclusions\u003c/h2\u003e\u003cp\u003eBased on systematically collected data from our regional birth registry, this cohort study confirmed the association between established risk factors for obstetric anal sphincter injury, such as instrumental vaginal delivery. However, we could not identify amniotomy as a risk factor for obstetric anal sphincter injury, possibly due to changes in clinical practice and potential data limitations.\u003c/p\u003e","manuscriptTitle":"Risk factors for obstetric anal sphincter injury in a Norwegian Region: a cohort study","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-08-26 12:38:49","doi":"10.21203/rs.3.rs-7299238/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-08-26T13:05:03+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-08-25T19:44:43+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"280799633494941322202950159419160307620","date":"2025-08-25T11:15:43+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"14051650700118998914235859418597293827","date":"2025-08-24T10:07:13+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"319779114118740539866212743681277801608","date":"2025-08-23T22:21:19+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-08-23T12:39:32+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"61317227979331876121798507912857445746","date":"2025-08-19T16:19:09+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-08-18T11:14:58+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2025-08-06T14:04:08+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-08-05T23:41:20+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-08-05T23:40:27+00:00","index":"","fulltext":""},{"type":"submitted","content":"BMC Pregnancy and Childbirth","date":"2025-08-05T09:53:23+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"bmc-pregnancy-and-childbirth","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"prch","sideBox":"Learn more about [BMC Pregnancy and Childbirth](http://bmcpregnancychildbirth.biomedcentral.com/)","snPcode":"","submissionUrl":"https://www.editorialmanager.com/prch/default.aspx","title":"BMC Pregnancy and Childbirth","twitterHandle":"@BMC_series","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"em","reportingPortfolio":"BMC Series","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"039959b9-2cae-4e42-b3d6-fbffb1916b74","owner":[],"postedDate":"August 26th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-12-29T16:09:42+00:00","versionOfRecord":{"articleIdentity":"rs-7299238","link":"https://doi.org/10.1186/s12884-025-08422-1","journal":{"identity":"bmc-pregnancy-and-childbirth","isVorOnly":false,"title":"BMC Pregnancy and Childbirth"},"publishedOn":"2025-12-25 15:58:01","publishedOnDateReadable":"December 25th, 2025"},"versionCreatedAt":"2025-08-26 12:38:49","video":"","vorDoi":"10.1186/s12884-025-08422-1","vorDoiUrl":"https://doi.org/10.1186/s12884-025-08422-1","workflowStages":[]},"version":"v1","identity":"rs-7299238","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7299238","identity":"rs-7299238","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}