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There is a widespread belief that sex during the later stages of pregnancy can jumpstart labour, but that doesn't appear to be so - at least according to a study from Malaysia.
The researchers, whose work appeared in BJOG: An International Journal of Obstetrics and Gynaecology, found that there were no differences in the timing of delivery between women who had sex near term and those who abstained.
"We are a little disappointed," said 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 labour a little earlier if (they) wanted."
Tan said that many women believe intercourse can induce labour, 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 labour. Breast stimulation is also thought to hasten labour and orgasm can trigger uterine contractions.
"labour induction for prolonged pregnancy is common and many women are also tempted for a variety of personal reasons to trigger labour in the very later stages of pregnancy," Tan said.
The researchers invited more than 1,100 women to participate, all of whom were 35 to 38 weeks pregnant 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 labour. The other half were told that sex was safe during pregnancy, but that its effects on labour were unknown.
The researchers then tracked the women to determine how long their pregnancies lasted and whether they required any medical intervention to start labour.
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 labour were similar in both groups: 22 percent of those advised to have sex and 20.8 percent of the other group, a difference so small it is likely to have been driven by chance.
Earlier research relied primarily on surveys of women about their sexual experiences during pregnancy, but this study was "the first attempt to really randomise the experience, for some to have sex and some to not, which is a very hard thing to do," said Jonathan Schaffir, an associate professor of obstetrics and gynecology at the Ohio State University College of Medicine.
"Even though this study did not show any increase in the rate of labour 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.