Some prenatal care providers advise pregnant women to abstain from sex, especially during the third trimester, to reduce risk of premature delivery. But does sex during late pregnancy increase risk of prematurity? Research findings are all over the map. Some studies show that third trimester sex does, indeed, increase risk of premature labor. Others show it has no effect. And one suggests that sex during late pregnancy decreases risk of prematurity. So what should a pregnant couple do?
Prematurity: High Risk of Complications
Pregnancy usually lasts about 40 weeks. Doctors generally define prematurity as delivery more than three weeks prior to the baby’s due date, or before around the 37th week.
Prematurity is associated with a host of problems. Short-term hazards to the infant include: infections, anemia, jaundice, low blood sugar (hypoglycemia), breathing problems, heart defects, bleeding in the brain, and digestive disorders. Longer-term risks as preemies grow include cerebral palsy, learning disabilities, and behavior, vision, hearing, and dental problems.
Maternal risk factors for prematurity include: diabetes, smoking, poor nutrition, multiple fetuses, a previous preemie, high blood pressure, some physical injuries, an interval of less than six months between pregnancies, in vitro fertilization, insufficient weight gain during the pregnancy, several infections, stressful life events (particularly domestic violence)—and according to some studies, sex, especially during the third trimester.
Many of these risk factors are beyond women’s control. Prenatal care providers urge pregnant women to avoid those that are preventable by not smoking, eating a plant-based diet, managing weight, and getting regular prenatal care. Some providers also advise abstaining from sex.
The Research: Confusing
I searched the medical literature and identified 10 studies since 1990 that have investigated sex as a possible risk factor for prematurity. Six show no association. Four do.
Studies showing that sex does NOT boost prematurity risk (and just might reduce it):
• Malaysian researchers instructed 108 women with normal pregnancies to have sex as much as they liked, but told 102 similar women to abstain. The two groups showed no difference in rate of prematurity.
• University of Texas scientists tracked the pregnancies of 187 women who’d had previous preemies, placing them at high risk of premature deliveries the next time around. During their subsequent pregnancies, sex made no statistically significant difference in their risk.
• Researchers at the University of Pittsburgh surveyed 341 women who’d delivered prematurely, asking about all their possible risk factors. Sex had no impact on risk.
• University of Colorado investigators followed 126 African-American teens through their pregnancies. Sex was not associated with prematurity.
• In a large, multicenter trial, scientists at the National Institutes of Health found that sex during pregnancy reduced risk of prematurity—unless the women had vaginal infections (trichomonas, mycoplasma), in which case the infections increased risk.
• Researchers with the National Institute of Environmental Health followed the pregnancies of 596 North Carolina women and found sex during pregnancy linked to a substantially reduced risk of prematurity.
Studies asserting that sex IS a risk factor for prematurity:
• University of Chicago investigators found that sex during pregnancy raised risk of prematurity—but only sex in the man-on-top (missionary) position.
• Researchers at Harvard analyzed the health and lifestyle of 50 women who’d experienced premature labor. Third trimester sex significantly increased risk.
• Chinese scientists compared 1391 women who’d experienced prematurity with 1391 women who’d delivered at term. Late pregnancy sex raised risk.
• Greek researchers analyzed all 5,250 births at one hospital, of which 351 were preemies. Sex during the last weeks of pregnancy increased risk.
What Should Pregnant Couples Do?
The weight of the evidence suggests that sex is not a major risk factor for prematurity; if women have no vaginal infections and pregnant couples avoid the man-on-top position the "score," at least among these studies, is eight to two.
But we can’t entirely dismiss the evidence showing a sex-induced prematurity risk, so I urge pregnant couples to review their risk factors for prematurity in consultation with their prenatal providers. The more risk factors you have, the more sexual caution your provider is likely to recommend.
If you’ve had a previous preemie and you have other risk factors, it’s probably prudent to abstain from sex during the third trimester and maybe earlier. But if you have a normal pregnancy and no risk factors, the evidence suggests that it’s fine to make love right up to term if you want to.
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Harger, JH et al. “Risk Factors for Preterm Premature Rupture of Fetal Membranes: A Multicenter Case-Control Study,” American Journal of Obstetrics and Gynecology (1990) 163:130.
Hernandez-Diaz, S et al. “Triggers of Spontaneous Preterm Delivery: Why Today?” Paediatric and Perinatal Epidemiology (2014) 28:79.
Petridou, E et al. “Are There Common Triggers of Preterm Deliveries?” BJOPG [formerly British Journal of Gynecology] (2001) 108:598.
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Stevens-Simon, C et al. “Factors Associated with Preterm Delivery Among Pregnant Adolescents,” Journal of Adolescent Health (1993) 14:340.
Tan, PC et al. “Effect of Coital Activity on Onsent of Labor in Women Scheduled for Labor Induction: A Randomized Controlled Trial,” Obstetrics and Gynecology (2007) 110:820.
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