A study from the University of Malaya in Malaysia found no differences in the timing of delivery between women who had sex near term and those who abstained, the Daily Mail reported.
“We are a little disappointed that we did not find an association,” the paper quoted author Dr Tan Peng Chiong as saying.
However, the team said having sex is usually perfectly safe before the woman’s waters have broken.
Dr Chiong said in the past scientists have proposed a number of plausible biological explanations for why sex could induce labour.
For one, semen contains a hormone-like substance called prostaglandin, which is used in synthetic form to induce labour. It was also suggested that orgasms could trigger uterine contractions.
However, few studies have ever investigated whether sex really can start the birthing process and the small amount of existing evidence has been inconclusive.
In the latest study, published in the journal BJOG, the team set out to see whether advising women to have sex during the final weeks of pregnancy could help them avoid an induction.
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 doctor 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 the rates of induced labor were similar in both groups: 22 per cent of those advised to have sex and 20.8 per cent 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 Chiang 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.