Does maternal sexual activity during the last two months of pregnancy cause premature birth? No--not according to a new study reported in the February 2001 issue of Obstetrics and Gynecology. In fact, it may prevent premature birth.

Researcher Amy Sayle, MD, and colleagues interviewed 187 women who had recently given birth prematurely (between 29 and 36 weeks) about their sexual activities during pregnancy. Their answers were compared with answers from a control group of 409 women who gave birth at an average of 39.2 weeks (37 weeks being considered full-term). The researchers found that sexual intercourse and maternal orgasm during late pregnancy were actually associated with decreased risk of premature birth.

In the past, physicians have theorized that maternal orgasm could release the hormone oxytocin (OX-ee-TOH-sin), which causes the womb to contract. Some have speculated that certain compounds in semen (prostaglandins) also have oxytocic properties, meaning they could hasten the onset of labor. Others have expressed concern that penetrating vaginal intercourse could increase the risk of infection that might result in a premature birth.

While Sayle and colleagues found no evidence to support these concerns, the question is not completely closed, says Zane Brown, MD, a professor of obstetrics and gynecology at the University of Washington. "It all depends on the population," says Brown. "If you have a totally healthy population of pregnant women, you have no relationship between intercourse and pre-term labor. On the other hand, among the broader population, there are subsets of women in whom you find a cluster of risk factors for preterm labor, including smoking, high stress, poor nutrition, and abnormal bacteria in the genital tract."

And what's the specific concern about intercourse during late pregnancy in such higher-risk women? Perhaps most important is the risk of sexually transmitted diseases (STDs). STDs such as genital herpes and bacterial vaginosis can be acquired during pregnancy, and both are linked with adverse birth outcomes. Beyond this, however, Brown asserts that sex can introduce into the vagina a number of different microbes such as E. coli and group B streptococcus, and these can raise the likelihood of premature birth in combination with other factors.

As far as any inherent risk from sexual activity itself, the new study by Sayles et al suggests that prospective parents shouldn't worry, though the authors point out that "sexual activity might be a risk for a small subgroup of susceptible women." For those in this risk group, says Brown, regular condom use is the single best precaution.