"We are a little disappointed that we did not find an association," said Dr. Tan Peng Chiong, an obstetrics and gynecology professor at the University of Malaya, and one of the authors of the study. "It would have been nice for couples to have something safe, effective and perhaps even fun that they could use themselves to help go into labor a little earlier if (they) wanted."

Tan said that many women believe intercourse can induce labor, and scientists have proposed plausible biological explanations for why it might help.

For one, semen contains a hormonelike substance called prostaglandin, which is used in synthetic form to induce labor.

Breast stimulation is also thought to hasten labor and orgasm can trigger uterine contractions.

Tan said the belief also probably came from "the deep seated folk perception that intercourse in pregnancy may be unsafe and may cause pregnancy expulsion or miscarriage despite fairly replete evidence to the contrary."

But few studies have ever investigated whether sex really can initiate labor, and the small amount of existing evidence was inconclusive, Tan and his colleagues write in the obstetrics and gynecology journal BJOG.

So they set out to see whether advising women to have sex during the final weeks of pregnancy could help them avoid an induction.

"Labor induction for prolonged pregnancy is common and many women are also tempted for a variety of personal reasons to trigger labor off in the very latter stages of pregnancy," he said.

The researchers invited more than 1,100 women to participate, all of whom were 35 to 38 weeks pregnant (the typical pregnancy lasts 40 weeks) and none of whom had had sex in the previous six weeks.

Roughly half of the women were advised by a physician to have sex frequently as a means of safely expediting labor.

The other half were told that sex was safe during pregnancy, but that its effects on labor were unknown.

The researchers then tracked the women to determine how long their pregnancies lasted and whether they required any medical intervention to start labor.

They found that about 85 percent of the women who were encouraged to have sex did follow the doctor's advice, while 80 percent of women in the other group also had sex.

Women in the group advised to have sex also had it more frequently for the remaining duration of their pregnancies - three times versus two.

But the rates of induced labor were similar in both groups: 22 percent of those advised to have sex and 20.8 percent of the other group had inductions, a difference so small it is likely to have been driven by chance.

Pregnancy also lasted an average of 39 weeks for both groups.

Dr. Jonathan Schaffir, an associate professor of obstetrics and gynecology at The Ohio State University College of Medicine who has studied obstetrical folklore, said the findings offer good support for the idea that sex won't induce labor.

Earlier research had relied primarily on surveys of women about their sexual experiences during pregnancy, but this study was "the first attempt to really randomize the experience, for some to have sex and some to not, which is a very hard thing to do," he said.

Schaffir wouldn't advise his patients to have sex for the express purpose of going into labor, he added, but the study did indicate that there aren't any hazards to it.

"Even though this study did not show any increase in the rate of labor or a decrease in the rate of induction, it helped to cement the idea that having sex is probably safe if you want it," he told Reuters Health.

Tan said the results show that pregnancy evolved to be resistant to disruption.

"Human pregnancy has to be robust to a little adventure like intercourse and unfortunately for our purpose, it seems pretty robust to the very end," he said.

SOURCE: bit.ly/WxE4wk BJOG: An International Journal of Obstetrics and Gynaecology, online November 12, 2012.

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