Induction of labor occurs in approximately 20% of term pregnancies, and prolonged pregnancy (defined as a pregnancy of at least 41 weeks of gestation) accounted for 46% of these inductions in the United Kingdom.1 In addition, 79% of prolonged pregnancies resulted in labor induction.1 Sexual intercourse is commonly believed to hasten labor.2 A Cochrane Review on sexual intercourse for cervical ripening and induction of labor could identify only a small trial with 28 women, from which no meaningful conclusions can be drawn.3
The effect of coitus on preterm labor is uncertain. A decreased risk of preterm birth has been reported to be associated with having intercourse in later pregnancy4 and also with having orgasms.5 On the contrary, increased risk of preterm births is also linked to having preterm intercourse.6 Semen contains prostaglandin E,7 breast stimulation has been shown to hasten the onset of labor,8 and coitus and orgasm stimulates uterine activity,9 thereby accounting for the expectation that sexual activity at term may promote labor.
A literature search of PubMed was carried out on January 15, 2006, in all languages, using the search terms “coitus” or “sexual intercourse” and “labor induction,” “postdate” or “postterm,” looking for articles published between January 1966 and January 2006. No relevant article was found, indicating a paucity of data on coitus at term on length of gestation and induction of labor. We undertook a prospective longitudinal study based on diary keeping to determine the incidence of coitus at term and to estimate its effect on important clinical issues like length of gestation, labor induction for prolonged pregnancy, and mode of delivery.
MATERIALS AND METHODS
A prospective longitudinal study based on diary keeping was done to investigate the effect of coitus from 36 weeks gestation until birth on length of gestation, postdate pregnancy (defined as pregnancy beyond the estimated date of confinement), labor induction for prolonged pregnancy (which we defined as pregnancy of at least 41 weeks gestation), and mode of delivery.
Women attending antenatal clinic between December 2002 and August 2003 had their antenatal charts scrutinized. A total of 344 healthy women were identified and approached to participate in the study; 241 were recruited, and following drop outs and exclusions, 200 women were left for final analysis (Fig. 1). We selected only healthy women with uncomplicated obstetric history and straightforward pregnancies close to 36 weeks of gestation to minimize the risk of medically indicated deliveries and also to ensure no medical impediment to coitus in late pregnancy. Women with a history of threatened miscarriage, antepartum hemorrhage, suspected fetal growth restriction, hypertension, gestational diabetes, multiple pregnancies, and in breech presentation in their present pregnancy were excluded. No study women had a previous preterm or postterm birth. We did not exclude women with a history of previous early miscarriage. Gestational age had been confirmed by a first-trimester ultrasound assessment in all recruited women.
Women who agreed to participate in the study answered a short questionnaire on their perception of coital safety in late pregnancy to provide details about their husbands and also some personal data. They were given a diary to chart coitus (defined as vaginal intercourse with penile penetration) from 36 weeks of gestation and to submit the diary on a weekly basis when they attend their routine antenatal clinic appointments. We did not ask the women to record orgasm, ejaculation into the vagina, or breast stimulation during intercourse. Any woman who failed to submit her diary for 2 consecutive weeks was contacted by telephone to obtain data.
Previous reports have shown that between 36% and 56% of healthy women are sexually active in late pregnancy.4,10 According to a U.K. national report from 2002, 52.3% of women remained undelivered at 40 weeks of gestation.11 Sample size calculation, using an α of 0.05 and β of 0.8, assuming a 50% abstinence rate in late pregnancy, and factoring in a risk ratio (RR) of 0.6 for being undelivered at 40 weeks of gestation among sexually active women compared with abstinent women, determined that 190 women were needed. Assuming a 20% dropout rate, 238 women needed to be recruited.
Women who reached 41 weeks of gestation at our center without contraindication for vaginal delivery were routinely offered labor induction for prolonged pregnancy. Labor induction was usually scheduled close to 41 weeks and 3 days of gestation. Induction was carried out with vaginal prostaglandin (dinoprostone 3 mg) when the cervix was unfavorable or amniotomy and oxytocin infusion if the cervical dilatation was 3 cm or more and the presenting part was low.
Each woman was contacted by telephone after delivery to ascertain her date of delivery, mode of delivery, and labor induction status and for us to answer any of her queries related to the study. The information obtained was checked with the delivery suite birth register and any discrepancy investigated and resolved.
Institutional approval for the study was obtained and institutional guidelines followed. All study women gave consent.
Data were entered into SPSS 13 software (SPSS Inc, Chicago, IL), and Fisher exact test for 2 × 2 data sets, t test for means, analysis of variance, and multivariable logistic regression analysis were performed using this software. In addition, GraphPad Instat and Quickcalcs software (GraphPad Inc, San Diego, CA) were also used for calculations of relative risk and number needed to treat with 95% confidence intervals (CIs).12 Fisher exact test for categorical data sets greater than 2 × 2 was calculated using SISA software (Uitenbroek, DG; available at: http://home.clara.net/sisa/fiveby2.htm; retrieved April 21, 2006). P < .05 in any test was considered statistically significant, and all tests used two-sided results.
We approached 344 women for recruitment into the study. After taking into account decliners, dropouts, and exclusions, 200 healthy women with complete diaries were left for analysis (Fig. 1). All women in the study were married and living with their husbands. Of the 14 women who were excluded because of medical indications for expedited delivery, four were abstinent and ten had coitus at term, compared with 84 abstinent and 116 sexually active study women (P = .41). Of the 3 excluded women with prelabor rupture of membrane cases, two had coitus at term and one woman was abstinent.
Of the 116 (58%) women who had coital acts, the median number of coital acts was 4 (interquartile range 2). The characteristics of the study women are listed in Table 1. Women who had intercourse at term differed from abstinent women in ethnicity, educational attainment, occupation, partner's age, and their perception of coital safety in late pregnancy. After controlling for educational attainment and occupation, multivariable logistic regression analysis indicated that Malay ethnicity, younger husbands, and perception of coital safety remained independently associated with coitus.
The distributions of gestational age at delivery of women who had coitus at term and women who were abstinent are shown in Figure 2. Only 6.9% of sexually active study women remained undelivered at 41 weeks of gestation, compared with 29.8% of abstinent women (Fig. 2).
Women who had coitus at term had a mean reduction in gestational length of only 4.4 days (mean ± standard deviation gestational length 39.3 ± 1.1 weeks in sexually active versus 39.9 ± 1.2 weeks in abstinent women, P < .001), but this reduction meant that on univariable analysis they were less likely to go postdate (RR 0.59, 95% CI 0.42–0.85, P = .001), less likely to remain undelivered at 41 weeks of gestation (RR 0.37, 95% CI 0.20–0.69, P < .001), and less likely to require labor induction for prolonged pregnancy (RR 0.31, 95% CI 0.14–0.69, P < .001). After multivariable logistic regression analysis controlling for the women's ethnicity, education, occupation, perception of coital safety, and husband's age, coital activity remained associated with reductions in postdate pregnancy (adjusted odds ratio [AOR] 0.28, 95% CI 0.13–0.58, adjusted P = .001), being undelivered at 41 weeks of gestation (AOR 0.10, 95% CI 0.04–0.28, P < .001), and the requirement for labor induction indicated by prolonged pregnancy (AOR 0.08, 95% CI 0.03–0.26, P < .001). As coital frequency increased, mean length of gestation decreased and the RR of postdate pregnancies, of being undelivered at 41 weeks gestation, and of labor induction for prolonged pregnancy also decreased (Table 2).
After coitus, birth was most likely to happen within 2 days, with a probable continuing effect on delivery for up to 5 days (Fig. 3). The effect of coitus on promoting spontaneous labor and birth was fairly consistent across each week of term gestation period, with RR of 1.1–3.4 (Table 3). This finding suggested that, as onset of spontaneous labor peaks at 39–41 weeks,1 less frequent intercourse would be required at these gestations per spontaneous labor stimulated compared with at 36–38 weeks of gestation. At 39 weeks of gestation, five couples would have to have intercourse for one women to avoid reaching 41 weeks of gestation still undelivered (number needed to treat12 = 5, 95% CI 2.9–9.9). Similarly, at 39 weeks of gestation, five couples would need to have intercourse for one woman to avoid labor induction at 41 weeks of gestation for prolonged pregnancy (NNT = 5, 95% CI 3.3–10.3).
Among the 25 women who had labor induced at 41 weeks of gestation, the mean (± standard deviation) gestational age at birth was 41 weeks and 3 days ± 2 days. There was no perinatal mortality among the study women.
Coitus at term as a mode of initiating labor is a popular belief.2 This prospective study of straightforward maternities with confirmed gestational age showed reductions in gestational length, postdate pregnancy, and labor induction at 41 weeks of gestation for prolonged pregnancy in women who continued to have intercourse at term. Biological plausibility for the effect of intercourse was enhanced by the observations that labor promotion was evident throughout term gestation (Table 3), the peak occurrence of birth was within 24–48 hours of intercourse (Fig. 3), and as the frequency of intercourse increased, its effect was more marked (Table 2).
At 39 weeks of gestation, 5 (95% CI 3.3–10.3) couples would need to have intercourse for one woman to avoid reaching 41 weeks of gestation. Similarly, 5 (95% CI 2.9–9.9) couples would need to have intercourse to avoid one labor induction for reaching 41 weeks of gestation. This finding has important clinical implications because labor induction at 41 weeks of gestation is a common practice.1,13,14
Our study shows that being non-Malay, and in particular, being a Chinese woman, was associated with coital abstinence at term. This finding is in agreement with Fok et al15 who have shown that Chinese pregnant women had fewer sexual activities and less desire during pregnancy.
Eryilmaz et al,16 in a recent study of Turkish women, has shown associations between sexual activities during pregnancy and duration of marriage, parity, gravidity, and education level. In our study, none of the above factors were independently associated with coitus at term. We found that having a husband aged 30 years or more was an independent risk factor for coital abstinence at term, and this male factor is consistent with the finding that men are the main initiators of sexual activity in pregnancy.17
In our study, as expected, women who felt that coitus was not safe were much more likely to abstain from intercourse. Coital safety in pregnancy is a universal issue, with 40% of Nigerian,18 45.4% of Pakistani,17 49% of Canadian,19 and as many as 80% of Chinese15 women expressing safety concerns. Among our study women, 60 (30%) did not feel that intercourse at term was safe—a lower percentage than in previous studies.
Because we have restricted our study to healthy women without pregnancy complications and because we excluded from our analysis a small number of women who developed minor complications, our study did not address the issue of safety, apart from ascertaining that all study women were discharged home with their infants. Previous studies of intercourse in pregnancy suggest that complications are uncommon and minor in nature.15,19 However, in potentially compromised pregnancies, caution has to be applied because the effect of intercourse and orgasm has been described as being similar to an oxytocin contraction stress test.9
Our findings need to be confirmed by intervention studies. Any intervention based on such a complex issue as sexual intercourse is likely to be challenging to implement effectively, and the widespread safety concern of women would have to be allayed before the suggested intervention could be widely adopted. Coitus at term can be an effective method for promoting spontaneous labor at term, thereby reducing the need for labor induction at 41 weeks of gestation.
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