Characteristics of menstrual cycles with or without intercourse in women with no known subfertility

Human females are spontaneous ovulators, experiencing an LH surge and ovulation cyclically, independent of copulation ( Adams et al. , 2016 ; Pavličev and Wagner, 2016 ). However, evidence suggests that in some placental mammals, environmental influences, including copulation, impact the timing of ovulation ( Adams et al. , 2016 ; Pavličev and Wagner, 2016 ). Across various species, it is unclear whether copulation-induced ovulation is a result of physical stimulation of the genitalia during copulation, the endocrinological and neurological changes that comprise female orgasm ( Pavličev and Wagner, 2016 ; Rabinerson et al. , 2018 ), ovulation-inducing factors in seminal plasma or a combination of these factors ( Adams et al. , 2016 ; Pavličev and Wagner, 2016 ). Studies in humans have found changes in female reproductive hormones attributable to copulation ( Prasad et al. , 2014 ), but the literature lacks work to evaluate the specific relationship of sexual intercourse to menstrual cycle characteristics. In this study, we aimed to investigate menstrual cycle characteristics, including ovulation, follicular and luteal phase lengths, bleeding patterns, and cervical mucus secretion patterns between cycles with and without sexual intercourse in regularly menstruating, sexually active, heterosexual women, with no history of subfertility.

The combined data comprised 2564 menstrual cycles from 530 women with no known subfertility. The mean number of cycles contributed per woman was 4.8 ± 3.5 cycles (min: 1, 25th: 2, 50th: 4, 75th: 7, max: 15), including 158 (6.2%) conception cycles. Ninety-four women had only one cycle. Most women were <30 years of age (75.5%), with a mean age of 27.1 ± 4.1 years (min: 18, 25th: 24, 50th: 27, 75th: 29, max: 40). Most women were non-Hispanic white (88.1%), married/engaged (89.9%), college graduates (73.8%), employed (78.3%), with a professional level of occupation (50.6%) and had a household income at >200% of federal poverty level (76.4%) in the respective year. Most women (70.9%) were nulliparous ( Table II ). Demographic and reproductive characteristics, and medical history of study population (530 women without known subfertility, 1990–2013). Total 279, not available for Creighton Model MultiCenter Fecundability Study (CMFS, 251 women) (see Table I ). Most women (77.4%) had intercourse in all their cycles in the study, while 19.6% of women had a mix of some cycles with and without intercourse, and 3.0% of women had no cycles with intercourse. Among the cycles with intercourse, the mean number of days with sexual intercourse per cycle was 5.4 ± 3.6 (min: 1, 25th: 3, 50th: 5, 75th: 7, max: 24). In the adjusted analysis, cycles without sexual intercourse compared to cycles with intercourse had shorter mean cycle length (29.1 days (95% CI 27.6, 30.7)) versus (30.1 days (95% CI 28.7, 31.4) ( Table III ). There was a lower probability of having a long cycle (>35 days) (probability ratio (PR) 0.36 (95% CI 0.18, 0.72)) among cycles without intercourse ( Table IV ). Adjusted mean of self-reported cycle characteristics in 530 women, stratified by sexual intercourse (SI). a Linear mixed models were used to generate least square means. Adjusted for age, parity, partial breast feeding and recent use of oral contraceptives (see Table II ). Number of cycles and women per variable is available in Supplementary Table SI . Among cycles with follicular or luteal phase bleeding or spotting. Days of bleeding or spotting in follicular phase have been counted from the day after menses through ovulation day. 0.01 < P  <   0.05. P  <   0.01. Proportions and risk ratios for selected cycle characteristics without intercourse compared to cycles with intercourse (referent). a , b Number of cycles and women per variable is available in Supplementary Table SI . Generalized linear mixed models were used to generate risk ratios and 95% confidence intervals. Days of bleeding or spotting in follicular phase have been counted from after the end of menses through ovulation day. 0.01 < P  <   0.05. P  <   0.01. PR, prevalence ratio; SI, sexual intercourse. Among all cycles, 97.0% had a mucus peak day and were therefore considered ovulatory (95% CI 96.4, 97.7). We did not find any difference in the proportion of ovulatory cycles, comparing cycles without and with sexual intercourse ( Table IV ). The follicular phase length was essentially the same for cycles without and with intercourse, 18.5 days (95% CI 17.1, 20.0) versus 18.6 days (95% CI 17.4, 19.9) ( Table III ). The proportion of cycles with a short follicular phase (21 days) was 0.8% (95% CI 0.5, 1.2) and 17.7% (95% CI 16.2, 19.2), respectively ( Table IV ). Cycles without intercourse had a shorter luteal phase length, 10.8 days (95% CI 10.2, 11.5) versus 11.4 days (95% CI 10.9, 12.0) ( Table III ), and a higher probability of having luteal phase deficiency (<10 days), PR 1.31 (95% CI 1.00, 1.71). Similar effects were seen for luteal phase deficiency with a cut-off of <9 days or <8 days ( Table IV ). The mean duration of menstrual flow was essentially the same for cycles without and with intercourse, 6.1 days (95% CI 5.8, 6.4) and 6.2 days (95% CI 5.9, 6.5), respectively ( Table III ). In the adjusted analysis, cycles with no intercourse had a higher probability of having premenstrual spotting on the last day or last two consecutive days of the cycle, PR 2.16 (95% CI 1.10, 4.23), and (PR 2.15 (95% CI 1.09, 4.24)), respectively ( Table IV ). We did not find any significant differences between cycles without and with intercourse for the mean number of days with peak-type (estrogenic quality) mucus, 7.6 days (95% CI 6.6, 8.5) versus 7.5 days (95% CI 6.6, 8.3), or for the mean cervical mucus cycle score 8.5 (95% CI 7.7, 9.2) versus 8.5 (95% CI 7.9, 9.2) ( Table III ). However, in adjusted analysis, cycles with no intercourse had a higher probability of having ≤2 days of peak-type mucus, PR 1.49 (95% CI 1.03, 2.15) ( Table IV ). Supplementary Table SII summarizes all menstrual cycle characteristics across the entire study population, including mean and percentile distributions. We conducted sensitivity analyses, repeating all analyses limited to women who contributed cycles both without and with sexual intercourse in the study (n = 104 women, 733 cycles, of which 169 without intercourse). The findings from these analyses were all consistent with the primary analyses. Even with a smaller sample size, most of the findings remained statistically significant, including the greater likelihood of a luteal phase <10 days, cycles ≤2 days of peak-type mucus, and premenstrual spotting (data not shown).

We conducted a pooled analysis of three cohorts of heterosexually active couples who received instruction in the Creighton Model FertilityCare system (CrM) through CrM centers across USA and Canada: (i) ‘ Creighton Model Effectiveness, Intentions, and Behaviors Assessment ’ (CEIBA, 2009–2013, 17 CrM centers in 13 US states and Toronto, Canada; Stanford and Porucznik, 2017 ), a prospective cohort of women without known subfertility, aimed to evaluate and classify pregnancy rates and pregnancy intentions during use of the CrM; (ii) ‘ Time to Pregnancy in Normal Fertility ’ (TTP, 2003–2006, single CrM center in Utah; Stanford et al. , 2014 ), a randomized trial, which aimed to assess the impact of CrM use on time to pregnancy in couples of proven fertility trying to conceive; and (iii) ‘ Creighton Model MultiCenter Fecundability Study ’ (CMFS, 1990–1996, 6 CrM centers in four US states; Stanford et al. , 2003 ), a retrospective cohort of presumably fertile and subfertile women using CrM, aimed to assess the relationship between vulvar mucus observations and the day and cycle-specific probabilities of conception. For CEIBA and CMFS, women were required at entry to the study to be seeking to avoid pregnancy (i.e. using CrM for natural family planning to avoid pregnancy); however, they were able at any point during the study follow-up to seek pregnancy ( Stanford et al. , 2003 ; Stanford and Porucznik, 2017 ). All cohort studies were approved by the Institutional Review Board (IRB) at the University of Utah (CEIBA IRB 34487, TTP IRB 7042, CMFS IRB 5246). From CEIBA, all participants, from TTP, only participants in the CrM intervention group, and from CMFS, only the presumably fertile women met the initial requirement for our study, i.e. women age 18–40 years, no clinical history of subfertility or conditions that may be associated with subfertility, and not breast feeding (TTP and CMFS), or if breast feeding, not doing so exclusively (CEIBA). Having at least one normal menses since the last use of hormonal contraception was required by all these cohorts ( Nassaralla et al. , 2011 ; Girum and Wasie, 2018 ). Eligible couples contributed a daily diary (CrM chart) for at least one full cycle, and up to 1 year (CEIBA and CMFS) or 7 cycles (TTP). Otherwise, women contributed data until conception, initiation of hormonal contraception, study withdrawal, loss to follow-up or no longer meeting eligibility requirements ( Stanford et al. , 2003 , 2014 ; Stanford and Porucznik, 2017 ). The details of assembling the combined dataset have been published elsewhere ( Najmabadi et al. , 2020 ), and are summarized in Table I . All women provided informed consent before participating in the studies. Number of cycles and women excluded and included, by cohort: Creighton Model MultiCenter Fecundability Study (CMFS), Time to Pregnancy in Normal Fertility (TTP) and Creighton Model Effectiveness, Intentions, and Behaviors Assessment (CEIBA). Original cohorts data description has been published elsewhere. 1–5 CMFS and CEIBA had one common center: St John’s Mercy Hospital—St Louis, Missouri. These cycles had information about cycle length and estimated day of ovulation, but lacked information about daily bleeding, mucus or intercourse. Includes 158 (6.2%) conception cycles, dropped from some measurements. Stanford et al. (2003) . Stanford et al. (2014) . Stanford et al. (2017) . Najmabadi et al. (2020) . Najmabadi et al. (2021) . All these cohorts included women beginning to use the Creighton Model FertilityCare System, a fertility awareness-based method or natural family planning method ( Hilgers and Prebil, 1979 ; Stanford et al. , 2003 ; Tham et al. , 2012 ). Women using the CrM vaginal discharge recording system record observations for stretch, color and sensation, and the absence or existence of bleeding and bleeding intensity each day in a daily diary (CrM chart). Women are also instructed how to use these observations to identify the estimated day of ovulation (EDO), and the days of potential fertility ( Hilgers et al. , 1978 ; Nassaralla et al. , 2011 ; Tham et al. , 2012 ; Manhart et al. , 2013 ; Stanford et al. , 2014 ). This information can be used to time intercourse to avoid pregnancy or to try to conceive ( Duane et al. , 2022 ). Whether they seek to avoid pregnancy or to conceive, women are instructed to record each act of intercourse or genital contact daily. The use of barrier methods is discouraged; the use of lubricants is neither encouraged nor discouraged. All CrM charts were on paper and were collected by CrM teachers or study staff at least every month during the first 3 months, and at least every 3 months following ( Hilgers and Prebil, 1979 ; Najmabadi et al. , 2020 , 2021 ). Our primary outcomes were a series of menstrual cycle characteristics based on cycle phase lengths, bleeding characteristics and cervical mucus secretion characteristics. We defined cut-off points a priori based on prior research or clinical estimates, as reported in prior analyses ( Najmabadi et al. , 2020 , 2021 ). We used the peak day of cervical mucus as the EDO and also the last day of the follicular phase. The mucus peak day is the last day in the cycle of any mucus discharge which is clear, stretchy or lubricative (estrogen-stimulated qualities). The peak day as a marker for ovulation has been validated in numerous studies and in reference to serial follicular ultrasound and/or the urinary surge of LH ( Fehring, 2002 ; Porucznik et al. , 2014 ; Ecochard et al. , 2015 ; Stanford, 2015 ). All cycles in this analysis were reviewed by at least two experts to identify the peak day most likely to reflect the day of ovulation ( Najmabadi et al. , 2021 ). We considered cycles without a plausible peak day of cervical mucus to be anovulatory. Supplementary Table SI summarizes primary outcome cycle selection criteria based on ovulatory and/or conception status of the cycle, and the number of cycles and women eligible for each analysis. In the event of conception, subsequent cycles from that woman were censored. There were five CMFS and two CEIBA conception cycles (seven women) with an unknown peak day. To impute these peak days, we used the same ovulation day of the first previous cycle available ( Mikolajczyk and Stanford, 2006 ). For one woman with only one cycle, we used the population median peak day ( Najmabadi et al. , 2020 , 2021 ). The cycle length was defined as the number of days from the first day of menstrual bleeding, identified by the woman in the CrM chart, to the last day of the non-conception cycle before the start of the next menses ( Reed and Carr, 2000 ; Mikolajczyk et al. , 2010 ; Nassaralla et al. , 2011 ; Najmabadi et al. , 2020 , 2021 ). We defined follicular phase length as the number of days from the first day of menstrual flow through the EDO (inclusive) in ovulatory cycles. Thus, we considered the EDO as the last day of the follicular phase ( Reed and Carr, 2000 ; Nassaralla et al. , 2011 ; Najmabadi et al. , 2020 , 2021 ). The luteal phase included days from the first day after the EDO through the last day of menstrual cycle, among ovulatory non-conception cycles. The first day of each menstrual cycle was the day identified by the woman as the start of the menstrual flow, regardless of the amount of bleeding on that day. Women reported vaginal bleeding each day as heavy, moderate, light, very light (spotting), or brown or black bleeding. No specific guidelines or definitions were given to women for the codes of bleeding intensity. Very light flow and brown or black bleeding were combined and defined as spotting. If bleeding intensity varied through the day, the highest intensity for the day was used. We also calculated an estimate of the intensity of menstrual flow over the first 6 days of the cycle, using a published index ( Nassaralla et al. , 2011 ). All days with mucus that was clear, stretchy or lubricative (estrogen-stimulated qualities) were considered days of peak-type mucus ( Hilgers and Prebil, 1979 ; Fehring, 2002 ; Bigelow et al. , 2004 ; Ecochard et al. , 2015 ; Najmabadi et al. , 2021 ). Days with any mucus discharge that had none of the characteristics of peak-type mucus were considered days with non-peak mucus ( Najmabadi et al. , 2021 ). Each day of each cycle was classified as having peak-type mucus, non-peak mucus or dry (no mucus), with the exception of days with moderate or heavy bleeding. The quality of cervical mucus prior to and including the EDO was also assessed with an established mucus cycle score index. As the estrogen-stimulated quality of the mucus increases, the total score gets closer to the highest possible of 16 ( Hilgers, 1988 ; Nassaralla et al. , 2011 ). Potentially fertile days, i.e. days with a significant probability of pregnancy if intercourse were to occur on that day, included most days with any mucus before the peak day (with some exceptions for prolonged unchanging mucus patterns), and the 3 days following any mucus peak day ( Hilgers and Prebil, 1979 ; Najmabadi et al. , 2021 ). All dry days, except those that have non-menstrual spotting or occur within 3 days after a mucus peak day or within 3 days after non-menstrual bleeding or spotting, were considered as non-fertile days ( Hilgers and Prebil, 1979 ; Najmabadi et al. , 2021 ). In our analysis, the primary exposure, intercourse, refers to any cycle with at least one occurrence of vaginal-penile intercourse, regardless of timing during the cycle. We did not have adequate data to differentiate acts of intercourse with and without the use of barrier methods. In a sensitivity analysis, we repeated analyses restricted to women who had at least one cycle with and one cycle without intercourse. We conducted adjusted analyses for the following factors as potential confounders that could influence both the probability of intercourse and menstrual cycle function: age (<30 years versus ≥30 years), parity (nulliparous versus parous women with at least one live birth), partial breast feeding and use of oral contraceptives within 60 days prior to the first day of the cycle. We used descriptive statistics to summarize women’s cycle characteristics: cycle phase lengths, parameters of bleeding and cervical mucus, as we reported in prior descriptive analyses ( Najmabadi et al. , 2020 , 2021 ). We conducted stratified analyses of menstrual cycle parameters with sexual intercourse status. Linear mixed models were used to assess continuous parameters and generalized linear models using Poisson regression with robust variance were used to assess dichotomous outcomes, adjusted for the potential confounders. All statistical analyses were performed using SAS version 9.4 (SAS Institute Inc., Cary, NC). All studies were approved by the University of Utah IRB, as well as local site IRBs.

We investigated the relationship between sexual intercourse and menstrual cycle characteristics among 2564 cycles in 530 heterosexually active premenopausal women (median age 27), without any known subfertility. We found an association between no intercourse in the cycle and a shorter luteal phase, a higher likelihood of a luteal phase <10 (or <9 or <8) days, a higher likelihood of premenstrual spotting, and a higher probability of having ≤2 days of peak-type (estrogenic) cervical fluid. The differences in cycle parameters are relatively subtle and may not have direct clinical implications. Nevertheless, follicular development, as assessed by preovulatory follicular size in the ovary, is positively related to levels of estradiol before ovulation (which in turn influences cervical mucus quality), and it also positively influences corpus luteum function and levels of progesterone after ovulation (which in turn influences luteal phase length, and perhaps whether there is spotting before the menses; Blackwell et al. , 2013 , 2018 ; Abdulla et al. , 2018 ). Therefore, it may be worth considering whether in some instances, an absence of sexual intercourse may contribute to a short luteal phase, luteal phase defect, fewer days of peak-type mucus or premenstrual spotting. Like other investigators, we found a substantial variability of the luteal phase length overall ( Blaicher et al. , 1999 ; Duijkers et al. , 2005 ; Jones and Lopez, 2006 ; Fritz and Speroff, 2011 ). Cycles with fewer days of peak-type mucus have lower potential fecundability ( Stanford et al. , 2003 ; Bigelow et al. , 2004 ). We cannot exclude entirely the possibility that seminal fluid or arousal fluid associated with sexual intercourse could sometimes be recorded as cervical fluid. However, the CrM has instructions to distinguish arousal fluid and eliminate seminal fluid after intercourse ( Hilgers et al. , 2004 ). We also did not have any data for the use of lubricants. Cycles with no intercourse had a significantly higher probability of premenstrual spotting. Speculatively, premenstrual spotting might be a marker for lower fecundability, as it has also been associated with the presence of endometriosis ( Heitmann et al. , 2014 ). In a prospective cohort of 259 regularly menstruating women aged 18–44 years with self-reported vaginal-penile intercourse in 1–2 cycles (the BioCycle Study), Prasad et al. found associations between any intercourse and higher levels of progesterone, estradiol and midcycle LH, but only when compared to women who reported never having had sexual intercourse. They also found that estrogen, LH and testosterone levels were higher on days of intercourse and the day before, but not the day after. They suggested their findings could be interpreted in both directions for causality: higher hormones increasing the probability of sexual intercourse, and/or sexual intercourse (or at least ever having sexual intercourse) increasing the level of reproductive hormones ( Prasad et al. , 2014 ). While our study population included only sexually experienced women, our findings could also be consistent with an influence in either direction between sexual intercourse, reproductive hormones and impacts of the hormones on luteal phase length, premenstrual spotting and days of estrogenic mucus ( Ecochard et al. , 2017 ; Richards, 2018 ). However, due to other non-biological precursors that can influence human sexual behavior ( Salonia et al. , 2010 ), assessing hormonal influences on female sexuality is complex. Presumably constant male sexual interest across the female cycle may mask the effects of cyclic changes in female sexual desire that might be triggered by her hormones ( Caruso et al. , 2014 ). We did not find an increased probability of sporadic anovulatory cycles (based on cervical mucus peak day) among cycles with compared to those without sexual intercourse. This finding is in agreement with the BioCycle Study (where ovulation was defined by peak serum progesterone ≤5 ng/ml and no observed serum LH peak) in cycles without or with intercourse among sexually active participants. However, in the BioCycle study, sexually active women compared to sexually inactive women had lower odds of sporadic anovulation (adjusted risk ratio 0.34 (CI 0.16–0.73); Prasad et al. , 2014 ). This study has some limitations. Our study participants were geographically dispersed but relatively homogeneous with regards to race, ethnicity, income and educational levels, and all had male partners, which may limit the generalizability of the findings. A potential confounding factor would be acute illness or stressful events that might reduce the events of intercourse during a cycle to zero, and also might disturb or alter cycle characteristics. Only one of the cohorts (TTP) had information noted systematically for daily stress and stressful events; in a prior analysis of that cohort, higher stress did not cause reduced fecundability, but the impact on cycle characteristics was not specifically studied ( Park et al. , 2019 ). In cycles with unrecognized conception and very early pregnancy loss, there is a possibility of an apparently longer luteal phase. We cannot rule this out as a possible explanation for a longer luteal phase in cycles with intercourse ( Wilcox et al. , 1999 ; Promislow et al. , 2007 ). However, this would not be a possible explanation for any changes in cervical fluid secretion. Although the cervical mucus peak day is a reliable marker of ovulation, it is not as precise as serial follicular ultrasound, or some hormonal measures, which introduces imprecision in the outcome measures ( Ecochard et al. , 2001 ; Stanford et al. , 2020 ). We cannot exclude the possibility of undetected subfertility or related gynecologic disorders among some of the women, such as undetected endometriosis or polycystic ovary syndrome, which would impact generalizability of our findings. However, it is not clear whether or how including more cycles from subfertile women would impact our findings; the generalized linear model does account for the number of cycles contributed per woman. Some cycles in the no intercourse group may have actually had undocumented intercourse or other sexual activity, but this would bias our results toward the null. The CrM discourages use of barrier methods, so we believe that most instances of intercourse involved exposure to semen. However, we did not have information to detect consistently cycles where barriers may have been used. Some studies suggest that seminal plasma may have an impact on female reproductive function ( Robertson and Sharkey, 2016 ; Hopkins et al. , 2017 ). Further research assessing the impact of seminal fluid exposure is recommended. Our dataset lacks any information about the occurrence of female orgasm, precluding our ability to evaluate the independent or combined impact of female orgasm on cycle characteristics. From an evolutionary perspective, human female orgasm does not seem to be essential for ovulation and reproductive success. Still, clitoral stimulation and female orgasm may have some residual influence on ovulation timing in humans ( Pavličev and Wagner, 2016 ). In a recent small study of 11 healthy women, using neuro-imaging technics, researchers showed that compared to a resting state, orgasm increases blood supply and elevates pituitary activation, which leads to higher plasma concentrations of oxytocin and prolactin ( Blaicher et al. , 1999 ), which may facilitate ovulation and enhance sperm and oocyte transport ( Huynh et al. , 2013 ). This large study has some key strengths, including prospectively collected data on bleeding, cervical mucus and intercourse in daily diaries. The inclusion of multiple cycles per woman in the study allowed us to assess within-woman variability across cycles without and with intercourse ( Harlow and Ephross, 1995 ). The standardized protocol for evaluating daily bleeding and cervical fluid (CrM), allowed us to describe detailed menstrual characteristics simultaneously, and our sample size allowed adjustment for age, parity, recent hormonal contraceptive use and breast feeding.

We found evidence of cycle characteristics suggesting lower fecundability among cycles with no intercourse, compared with cycles with intercourse, among sexually active women without any known subfertility. These included shorter luteal phases, more cycles with luteal phase <10 days, more cycles with premenstrual spotting, and more cycles with two or fewer days of peak-type mucus (high-quality estrogen-stimulated cervical fluid). Sexual activity may change reproductive hormonal patterns, and/or levels of reproductive hormones may influence the likelihood of sexual activity. Future work may help understand the extent to which exposure to seminal fluid, and/or female orgasm and/or timing of intercourse could impact menstrual cycle function. In theory, large data sets from women using menstrual and fertility tracking apps could be informative if women can be appropriately incentivized to record intercourse completely ( Bull et al. , 2019 ; Faust et al. , 2019 ). It is also of interest to understand how cycle characteristics may differ in women with gynecological problems or subfertility.

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