Management of Threatened Preterm Labor Based on Cervical Length: A Randomized Controlled Trial

Management of Threatened Preterm Labor Based on Cervical Length: A Randomized Controlled Trial | Authorea try { document.documentElement.classList.add('js'); } catch (e) { } var _gaq = _gaq || []; _gaq.push(['_setAccount', 'G-8VDV14Y67G']); _gaq.push(['_trackPageview']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })(); Skip to main content Preprints Collections Wiley Open Research IET Open Research Ecological Society of Japan All Collections About About Authorea FAQs Contact Us Quick Search anywhere Search for preprint articles, keywords, etc. Search Search ADVANCED SEARCH SCROLL This is a preprint and has not been peer reviewed. Data may be preliminary. 23 January 2026 V1 Latest version Share on Management of Threatened Preterm Labor Based on Cervical Length: A Randomized Controlled Trial Authors : Supakorn Chaiyakarn , Puntabut Warintaksa 0000-0002-7946-3774 , Pracha Nuntnarumit , Buranee Swatesutipun , and Piya Chaemsaithong [email protected] Authors Info & Affiliations https://doi.org/10.22541/au.176915895.54989537/v1 254 views 91 downloads Contents Abstract Supplementary Material Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract Objective : To evaluate whether the use of cervical length in managing threatened preterm labor can reduce inappropriate treatment. Design: A randomized controlled trial Setting: Delivery Unit, Bangkok, Thailand Population: Threatened preterm labor patients between 24 and 33 +6 weeks of gestation with a transvaginal cervical length between 15 and 30 mm. Methods: The patients were randomized into two groups: 1) the expectant group, and 2) the treatment group. The expectant group received no treatment, while the treatment group received standard corticosteroids and tocolytic agents. Main Outcome Measures: The primary outcome was inappropriate treatment, defined as undelivered for more than a week after a complete course of corticosteroid treatment or spontaneous preterm delivery within 7 days of recruitment without corticosteroid administration. Results : 25 were randomized to the expectant group and 23 to the treatment group. The rate of inappropriate treatment was significantly lower in the expectant group compared to the treatment group (8.0% vs. 95.7%; p<0.001; relative risk 0.08). There were significantly lower rates of maternal tachycardia, hypotension, and hypokalemia in the expectant group (p=0.03 for all). The expectant group also had a significantly shorter median of maternal hospital stays (days) [1 vs. 2; p< 0.01]. Conclusion : Patients with threatened preterm labor whose cervical length is within 15-30 mm can be safely managed expectantly. This approach significantly reduces the rate of inappropriate treatment, maternal side effects, and length of maternal hospital stays. Funding: Partially supported by Mahidol University (Strategic Research Fund 2025) and the Faculty of Medicine Ramathibodi Hospital (project ID R1106801007), to Piya Chaemsaithong. Introduction Spontaneous preterm birth is the leading cause of neonatal morbidity and mortality globally, including in high-income countries with advanced healthcare systems (1) . The etiology of spontaneous preterm birth is multifactorial (2) . Despite recognizing its risk factors, such as a history of preterm delivery, cervical surgeries, and previous miscarriage, spontaneous preterm birth frequently occurs in women without any identifiable risk factors (3) . In modern obstetric practice, women presenting with symptoms of threatened preterm labor are often overtreated despite evidence indicating that only 5-13% will subsequently deliver preterm (4-6) . The standard clinical management of threatened preterm labor typically involves hospital admission, along with the administration of antenatal corticosteroids and tocolytic agents. While antenatal corticosteroids effectively improve neonatal outcomes, multiple courses of administration are associated with short- and long-term adverse consequences, including low birth weight, shorter height stature, smaller head circumference, and neurodevelopmental delay (7-13) . Notably, neurologic deficits have been reported to persist up to 5½ years of age (13) . Similarly, the use of tocolytic agents is associated with dose-related maternal complications such as tachycardia, hyperglycemia, and pulmonary edema (14-17) . Furthermore, the overdiagnosis of preterm labor contributes to increased healthcare expenditure, maternal stress, and disruption of family dynamics (4, 18-21) . Therefore, it is clinically imperative to accurately identify women at true risk of imminent preterm delivery to optimize treatment and avoid unnecessary interventions. Currently, digital assessment of the cervix is a standard tool for risk assessment of preterm birth. However, this method is invasive and often considered unreliable due to its low predictive value, subjectivity, and poor reproducibility (19, 22-25) . Existing research has identified transvaginal sonographic measurement of cervical length as a more promising alternative tool for predicting spontaneous preterm birth (26-30) . This tool is non-invasive, reproducible, and available in most healthcare settings. A substantial body of evidence demonstrates that in asymptomatic women, a mid-trimester cervical length of <25 mm serves as a powerful predictor of spontaneous preterm delivery (31-35) . In women presenting with symptoms of preterm labor, two cervical length thresholds have been established to provide clinical utility. Firstly, a cervical length ≥30 mm is associated with a high negative predictive value, suggesting that delivery within 7 days is improbable (36-41) ; these patients may be safely discharged without intervention (19, 42, 43) . In contrast, a cervical length 89% for predicting delivery within 7 days, indicating a high likelihood of imminent preterm birth (41, 44-47) . However, in the case of patients with threatened preterm labor where cervical length is measured between 15 and 30 mm, the management remains uncertain and controversial (48, 49) . The objective of this study is to evaluate whether the cervical length measurement can guide the management of women presenting with threatened preterm labor, aiming to reduce inappropriate treatment and unnecessary corticosteroid administration in those who do not deliver within 7 days. Methods A randomized controlled trial was conducted to recruit patients with threatened preterm labor between 24 and 33 +6 weeks of gestation who were admitted to the Labor and Delivery unit at the Department of Obstetrics and Gynecology, Faculty of Medicine, Ramathibodi Hospital, between January 24, 2024, and May 30, 2025. The inclusion criteria were singleton pregnancies with regular uterine contractions \(\geq\)4 times in 30 minutes (39) and a transvaginal cervical length of 15-30 mm. We excluded those with cervical length 30 mm, multiple gestations, premature rupture of membranes, suspected cervical insufficiency, or conditions that required imminent delivery, such as unstable maternal condition, chorioamnionitis, preeclampsia with severe features, active vaginal bleeding, placental abruption, or non-reassuring fetal heart rate. We also excluded patients who received corticosteroids or tocolysis within 7 days before hospital admission, as well as those who participated in this study and had a hospital readmission. Written informed consent was obtained from all participants, and the study was approved by the Ramathibodi Institutional Review Board (COA.MURA 2023/963). This study was registered in the Thai Clinical Trials Registry, with registration number TCTR20250218004. Study protocol Participants were randomized into two groups: 1) the expectant management group and 2) the treatment group. The randomization process utilized a computer-generated system with a block size of four and a 1:1 ratio. The allocated groups were concealed in serially numbered, sealed, and opaque envelopes that were opened only after recruitment, in the presence of a witness or third party. In the treatment group, participants received tocolytics and antenatal corticosteroids as per hospital protocol. In contrast, in the expectant group, participants were managed expectantly without tocolysis and antenatal corticosteroids. The measurements of cervical length were performed every 2 hours if there were regular uterine contractions with intervals of less than 10 minutes or every 4 hours if the contractions occurred at intervals >10 minutes or were irregular until there was no uterine contraction (Figure 1) . Participants in this group were allowed to receive sedative drugs, and tocolytic or antenatal corticosteroid agents were only required when: 1) the cervical length is shortening by more than 20% compared to the baseline value, or 2) cervical length shortened to <15 mm (18) , or 3) persistent uterine contractions ≥8 hours after the recruitment. Participants readmitted within 7 days of recruitment continued to receive care according to the group to which they had been initially randomized. The randomization and follow-up were performed by S.C., P.W., and P.C. Standard management of preterm labor Patient management per hospital protocol was guided by digital cervical assessment and uterine contractions, and they were administered corticosteroids, tocolytics, and antibiotics. The first line of tocolytic drug is nifedipine orally (10 mg every 15 minutes, maximum dose 40 mg/hr). If uterine contractions persist or there is cervical shortening, intravenous terbutaline (5 micrograms/min titrated every 15 minutes, maximum dose 25 micrograms/min) is administered. Given an unknown Group B Streptococcus status, antibiotics (ampicillin 2 g intravenously every 6 hours) were administered for 48 hours. For the induction of fetal lung maturity, dexamethasone 6 mg intramuscular for four doses was administered in cases with a gestational age <37 weeks (50) . In those with a gestational age neuroprotection before delivery. The patients were delivered if advanced labor, clinical chorioamnionitis developed, or if a non-reassuring fetal status developed. Ultrasound measurement of cervical length Upon admission to the hospital, before the digital and sterile speculum examination, a transvaginal ultrasound examination was conducted by trained operators (maternal-fetal medicine fellows or specialists) accredited by the Fetal Medicine Foundation (www.fetalmedicine.org) to measure cervical length. Briefly, all women had an empty bladder and were in a modified lithotomy position. The 2-dimensional transvaginal ultrasound transducer (GE HealthCare, based in Chicago, Illinois, USA) was introduced into the anterior fornix. After identifying the entire length of the endocervical canal in the sagittal plane, the position and pressure of the transducer were adjusted, and the ultrasound image was magnified to ensure that the cervix occupied 50% to 75% of the image. Care was taken to visualize the endocervical canal clearly, without excessive pressure on the cervix to avoid artificial lengthening. Cervical length was then measured by the linear distance between the internal and external cervical os, with each measurement performed three times, and the shortest distance was selected. The measurement was performed in the absence of uterine contraction. Study outcomes The primary outcome was inappropriate treatment, defined as delivery more than 7 days after the complete course of antenatal corticosteroids was given or spontaneous preterm delivery within 7 days of recruitment, but corticosteroids were not provided. The secondary outcomes included the rates of delivery within 7 days and 14 days of complete course of corticosteroids administration, the rates of spontaneous preterm delivery <34 and <37 weeks of gestation, gestational age at delivery, the rates of the patients who received therapy (tocolytic and antenatal corticosteroids), duration of maternal hospitalization, maternal side-effects from tocolytic drugs such as hypotension (blood pression <90/60 mmHg), tachycardia (pulse rate mEq/L), and pulmonary edema. We also evaluated neonatal outcomes such as birth weight, neonatal intensive care unit (NICU) admission, neonatal morbidity (respiratory distress syndrome, early onset neonatal sepsis, intraventricular hemorrhage grade III-IV), and mortality. Maternal characteristics and pregnancy outcome Maternal demographic characteristics, including age, weight, height, body mass index (BMI), smoking habit, parity, medical, surgical, and obstetrical history, including chronic medical conditions (e.g., hypertension), medications, history of prior preterm delivery, and prior preeclampsia, were collected at enrollment. Obstetric and delivery outcomes, including gestational age at delivery, the onset of labor, frequency of uterine contractions, cervical parameters, the use of antenatal corticosteroids or tocolytic agents, pain control medication, mode of delivery, duration of labor, and maternal hospitalization, and maternal and neonatal outcomes including complications were obtained from the maternity computerized records and kept in our secure database. Clinical definitions Pregnancy dating was performed using standard fetal biometry ultrasonography in the first or second trimester, according to the guidelines from the American College of Obstetricians and Gynecologists (ACOG) (51) . Threatened preterm labor is defined as pregnancies presenting with symptoms of preterm labor (such as uterine contractions ≥4 times in 30 minutes) with cervical dilatation <2 cm and intact membranes (52-54) . Preterm delivery is birth between 20 weeks and 36 6/7 weeks of gestation (52) . Inappropriate treatment is defined as delivery occurring more than 7 days after the completion of a corticosteroid course, or spontaneous preterm delivery within 7 days of recruitment without corticosteroid administration. Statistical analysis The sample size calculation was derived from a previous study reporting that 90% of patients receiving corticosteroids did not deliver within 7 days (47) . We hypothesized that the management of preterm labor based on cervical length would reduce unnecessary treatment (defined as undelivered after 7 days of a complete course of antenatal corticosteroid administration) by at least 50%. At a power of 80%, a type I error rate of 0.05, and a 10% loss to follow-up, a minimum of 21 cases with threatened preterm labor per group was required. The normality of the data was tested using the Kolmogorov-Smirnov test. Descriptive data were presented as mean (standard deviation) or median (interquartile range, IQR) for continuous variables and as numbers and percentages for categorical variables. Comparisons between groups were performed using chi-square tests with the continuity correction, Fisher’s exact tests for categorical variables, and Student t-tests or Mann-Whitney U-tests for continuous variables. A p-value less than or equal to 0.05 is considered statistically significant. Data were analyzed according to the intention-to-treat principle using the Statistical Package for the Social Sciences (SPSS) Version 29.0.2.0 (IBM Corp., Chicago, USA). Results Characteristics of the study population During the study period, 105 patients with threatened preterm labor were identified. Among these, 57 cases were excluded due to the following reasons: unmet cervical length criteria (cervical length <15 or dexamethasone within 7 days before recruitment (n=4). Therefore, 48 patients were recruited in this study (25 cases in the expectant group and 23 cases in the treatment group) (Figure 2) . There were no differences in median (IQR) maternal age, BMI, or the frequency of prior preterm births between the two groups (Table 1) . The median (IQR) gestational age (weeks) or cervical length (mm) at the time of recruitment was also not statistically different between the two groups (p>0.05 for all) (Table 1) . The frequencies of spontaneous preterm delivery <34 and < 37 weeks of gestation were 16.7% (8/48) and 29.2% (14/48), respectively. Study outcomes Two patients in the expectant management group (2/25; 8.0%) received antenatal corticosteroids due to progressive shortening of cervical length. However, all cases in this group remained undelivered for more than 7 days after recruitment. In the treatment group, almost all cases (except one) remained undelivered within 7 days after antenatal corticosteroid administration. The rate of inappropriate treatment was significantly lower in the expectant group compared to the treatment group [8.0% (2/25) vs. 95.7% (22/23); p<0.001; relative risk 0.08; 95% CI: 0.022-0.317]. The expectant group had a significantly shorter median (IQR) duration of maternal hospital stay compared to the treatment group [1 day (1–1) vs. 2 days (2–3), p-value < 0.01]. In addition, the expectant group was associated with less tocolytic use [8.0% (2/25) vs. 100% (23/23); p-value <0.01] and maternal complications, including maternal tachycardia [4.0% (1/25) vs. 39.1% (9/23); p=0.03], hypotension [4.0% (1/25) vs. 26.1% (6/23); p=0.03] and hypokalemia [0% (0/25) vs. 17.4% (4/23); p =0.03] The median gestational age at delivery, as well as the rates of delivery within 7 and 14 days, and spontaneous preterm delivery at <34 and 0.05 for all; Table 2 ). Neonatal outcomes, including birth weight, the frequencies of low birth weight, NICU admission, and endotracheal tube insertion, were not significantly different between the two groups (Table 2) . Discussion Principal findings of this study: 1) The rate of inappropriate treatment was significantly lower in the expectant group compared to the treatment group; 2) patients with threatened preterm labor who had a cervical length between 15-30 mm without further cervical shortening do not require corticosteroids and tocolytic agents; 3) none of the patients with the cervical length between 15-30 mm delivered within 7 days without corticosteroids administration, therefore, significantly reduces the unnecessary corticosteroids and tocolytic drugs use; and 4) the management of threatened preterm labor based on cervical length contributes to a significant reduction in the rates of maternal complications due to tocolytic use such as maternal tachycardia, hypotension, hypokalemia and a shorter maternal hospital stay. Result in the context of what is known Cervical length measurement is a widely used tool to guide the management of threatened preterm labor. A cervical length <15mm has a sensitivity of 60-75% for the identification of spontaneous preterm delivery within a week, with a specificity of 90% (40, 41, 44-46) . Thus, this tool is considered a reliable short-term predictor for preterm delivery. Several professional organizations, including the National Institute for Health and Care Excellence (NICE), the Society for Maternal-Fetal Medicine (SMFM), and the International Society of Ultrasound in Obstetrics and Gynecology (ISUOG), recommend using cervical length to manage threatened preterm labor (48, 54, 55) . Specifically, according to the SMFM’s recommendation, a cervical length ≥30 mm had a high negative predictive value (96-100%), suggesting no intervention or treatment (55) . Women with a cervical length <20 mm require hospital admission and treatment (55) . The ISUOG recommends using a cervical length cut-off of 30mm for the expectant management (54) . Nevertheless, the management of preterm labor with a cervical length between 15 and 30 mm remains controversial (49) . To date, one randomized controlled trial has been conducted to determine whether the management of threatened preterm labor based on the transvaginal measurement of cervical length can reduce the rate of unnecessary corticosteroid and tocolysis treatment (47) . In this study, participants were randomized to routine care (i.e., received tocolytics and antenatal corticosteroids) or were allocated to the experimental group based on cervical length measurement. If the cervical length was <15 mm, tocolytics and antenatal corticosteroids were administered; if≥15 mm, they were managed expectantly without tocolysis or corticosteroids. The authors concluded that management of threatened preterm labor based on cervical length was associated with a significant reduction in the unnecessary use of corticosteroids, defined as delivery >7 days after the complete administration of corticosteroids (relative risk 0.09; 95% CI: 0.02-0.39) (47) . Our study differs from the above study in that we use cervical length for the treatment randomization. Our results highlight that patients with threatened preterm labor who had a cervical length between 15 and 30 mm do not require treatment, as none of these patients delivered within 7 days without corticosteroid administration. This finding allows for a significant reduction in the unnecessary use of corticosteroids and tocolytic agents. Indeed, only one patient in the treatment group was delivered within seven days after recruitment. Specifically, this patient was recruited into the treatment group at 31 weeks and 3 days of gestation, and her initial cervical length was 27.5 millimeters with a closed cervix. The patient was admitted for 48 hours and received a complete course of antenatal corticosteroids. Unfortunately, 72 hours after hospital discharge, the patient was readmitted due to uterine contractions, and her cervical length was 12 millimeters (56% reduction compared to the baseline value). Despite the administration of tocolytic agents, persistent uterine contractions and progressive cervical dilation were observed (2 centimeters). Cesarean delivery was performed due to a history of prior Cesarean delivery. Altogether, our results indicate that appropriately targeted interventions for patients who are impending delivery within 7 days are important since they lead to a reduction in maternal hospital stay and maternal complications associated with tocolytic treatment without jeopardizing neonatal adverse outcomes. Clinical implications We proposed that patients presenting with preterm labor whose cervical length is between 15-30 mm can be managed expectantly without the administration of tocolytic agents or corticosteroids. We have chosen the treatment criteria based on cervical length shortening of previous studies (18, 47) . There were two patients in the expectant management group who received dexamethasone and tocolytic agents due to cervical shortening of >20% and cervical length <15 mm. However, neither of these patients was delivered within 7 days of completing corticosteroids. Thus, we believe that the 15 mm cut-off for initiating treatment, although not yet well established, could be appropriate based on recommendations from professional organizations (53, 54) . Some studies reported using cervicovaginal fluid fibronectin, a biomarker of choriodecidua activation, in women with threatened preterm labor and cervical lengths between 15 and 30 mm (21, 49, 56-62) . However, evidence regarding the incremental benefit of adding fetal fibronectin to cervical length is contradictory (21, 49, 56-62) . The advantages of cervical length measurement over fetal fibronectin include lower cost and broader availability across most labor and delivery units. Research implications Challenges to clinical implementation include the need for a standardized protocol for follow-up of cervical length after expectant management. In this study, we measured cervical length every 2–4 hours, depending on the regularity and interval of uterine contractions. Yet, the optimal frequency of cervical length measurement after clinical presentation remains unclear. We propose determining the proper frequency and duration of cervical length measurement in future studies. Additionally, whether the 15 mm cut-off for initiating treatment is appropriate has not been well-established. Importantly, the causes of preterm labor, such as intraamniotic infection or inflammation (63, 64) , vascular disorders (65, 66) , or breakdown of maternal-fetal tolerance (67, 68) , have not been comprehensively investigated (2) . We believe that the results from our study provide a basis for future research, focusing on validating these findings across diverse populations and exploring the cost-effectiveness and practicality of implementing these approaches in routine clinical settings to optimize the management of threatened preterm labor. Strengths and limitations The primary strength of this study lies in its randomized controlled trial design, which features a rigorous and well-defined protocol. Cervical length was measured by an accredited research team. Additionally, no patients were lost to follow-up. The limitation is a small sample size, which undermines its ability to perform subgroup analysis. Yet, such a sample size is enough to determine the primary outcome. Due to the nature of the study and intervention, neither the clinicians nor the patients were blinded to the treatment. Additionally, most patients in this study had a cervical length of 20-30 mm (72.9%, 35/48), which may not represent the highest-risk subset for imminent preterm delivery. The data were derived from an Asian population cohort, which may limit the generalizability of the findings to other populations. However, the incidence of spontaneous preterm delivery 14/48), along with the demographic characteristics of participants with threatened preterm labor, was comparable to those reported in other studies (39, 43, 44, 49, 69, 70) . Conclusions Women with threatened preterm labor whose cervical length is within 15-30 mm without additional cervical shortening can be safely managed expectantly without tocolysis or steroids. This approach significantly reduces the rate of unnecessary treatment, maternal side effects, and length of maternal hospital stays. Acknowledgments: We would like to thank the nurses and physicians at the antenatal care unit and the labor and delivery unit at Ramathibodi Hospital, Mahidol University. Research ethics: The research report described herein has been carried out in accordance with the World Medical Association’s Declaration of Helsinki (Code of Ethics) for studies involving humans. The study was approved by the Institutional Research Board of the Faculty of Medicine, Ramathibodi Hospital, Mahidol University (COA.MURA 2023/963). This study was registered in the Thai Clinical Trials Registry, with registration number TCTR20250218004. Written informed consent was obtained from the patient for publication and the accompanying images. Informed consent: Written informed consent for publication was obtained from the patients. Author contributions: The authors have accepted responsibility for the entire content of this manuscript and approved its submission. Competing interests: The authors state that they have no conflicts of interest. Data availability: All data generated or analyzed are included in this published article. For further information, please contact the corresponding authors. Authors’ contributions Supakorn Chaiyakarn: principal investigator, research design, sample collection, initial sample processing, patient management, cervical length measurement, conceptualization of the study, obtaining funding, initialization of the study, statistical data analysis, data interpretation, result synthesis, synthesis of the results, critical result review and suggestion, literature review, original draft preparation, and manuscript review and editing, manuscript finalization. Puntabut Warintaksa: Patient management, cervical length measurement, the conceptualization of the study, initialization of the study, and manuscript finalization. Pracha Nuntnarumit: The conceptualization of the study, study design, critical result review, and suggestion, as well as the provision of intellectual comments, manuscript finalization. Buranee Swatesutipun: The conceptualization of the study, study design, critical result review, and suggestions provide intellectual comments and manuscript finalization. Piya Chaemsaithong: Supervision the principal investigator, research design, sample collection, initial sample processing, patient management, cervical length measurement, conceptualization of the study, obtaining funding, initialization of the study, statistical data analysis, data interpretation, result synthesis, synthesis of the results, critical result review and suggestion, literature review, original draft preparation, and manuscript review and editing, manuscript finalization. All authors agree with the manuscript. Tables Table 1: Characteristics of the study subjects Table 2: The study outcomes Figures legend Figure 1: The study protocol Figure 2: The randomized controlled trial CONSORT flow diagram References 1. Slattery MM, Morrison JJ. Preterm delivery. Lancet. 2002;360(9344):1489-97.2. 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Keywords delivery: perineal care labour: management maternal medicine paediatrics: neonatal preterm labour: basic science preterm labour: clinical research Authors Affiliations Supakorn Chaiyakarn Mahidol University Faculty of Medicine Ramathibodi Hospital View all articles by this author Puntabut Warintaksa 0000-0002-7946-3774 Mahidol University Faculty of Medicine Ramathibodi Hospital View all articles by this author Pracha Nuntnarumit Mahidol University Faculty of Medicine Ramathibodi Hospital Department of Pediatrics View all articles by this author Buranee Swatesutipun Mahidol University Faculty of Medicine Ramathibodi Hospital Department of Pediatrics View all articles by this author Piya Chaemsaithong [email protected] Mahidol University Faculty of Medicine Ramathibodi Hospital View all articles by this author Metrics & Citations Metrics Article Usage 254 views 91 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Supakorn Chaiyakarn, Puntabut Warintaksa, Pracha Nuntnarumit, et al. 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