Having sex during the later stages of pregnancy doesn't speed up the start of labour, a Malaysian study finds.

There's a folk belief that sexual intercourse helps induce labour, and scientists have reported several biological explanations:

• Semen contains prostaglandin, a hormone-like substance.
• Breast stimulation hastens the onset of labour.
• Coitus and orgasm stimulate uterine contractions.

But when obstetricians randomly assigned about 1,200 women who were between 35 to 38 weeks in to pregnancy to test the idea, it didn’t hold up to scientific scrutiny.

The experiment focused on women who were at least 35 weeks in pregnancy. (Jorge Silva/Reuters)

"Despite the increase in reported coitus, there was no observed impact on the onset of labour or the labour induction rate in our current trial," study author Tan Peng Chiong, an obstetrics and gynecology professor at the University of Malaya, and co-authors concluded in this month's issue of BJOG: An International Journal of Obstetrics and Gynecology.

In the experiment, a doctor advised about half of the women to have vaginal intercourse frequently as a natural way of safely expedite labour and that the more frequent the better.

Those in the control group were told they were participating in a diary-based study of coitus and that sexual intercourse is safe but its effect on labour was uncertain.

The study looked at women who were recently abstinent to look for the greatest impact, the researchers said.

About 85 per cent of those in the group who were advised coitus reported doing so compared with nearly 80 per cent in the control group, a difference that showed coital activity at term can be influenced by medical advice, the study's authors said.

They found no differences in length of labour, need for oxytocin, mode of delivery, postpartum hemorrhage or measures of the newborn's health at delivery, which suggest sex is safe at term.

While the study included more than 1,000 participants who were randomly assigned, it's possible that those in the control group found out about the purpose of the experiment by talking.

The researchers said only 12 per cent of the women returned the diaries. The rest of the reports of coital activity were taken over the phone, a delay that may have reduced the accuracy of their recall, the investigators said.

Last year, another study reported that half of postpartum women said they’d tried sexual non-prescribed methods of inducing labour, such as sexual intercourse and nipple stimulation — a reflection of the demand to shorten pregnancy, the researchers said.

The study was funded by the University of Malaya.