A new study calls into question the folk wisdom that having sex late in pregnancy can help bring on labor.

Women who had sex a few days before they were scheduled for labor induction were no more likely to go into labor spontaneously than women who didn’t have intercourse, Dr. Peng Chiong Tan of the University of Malaya in Kuala Lumpur, Malaysia and colleagues found.

But this study is not the final word on the subject, Tan cautioned. “Further research in this area is needed - particularly in a less ‘pressure cooker’ situation than where labor induction was already scheduled for shortly ahead,” Tan noted in comments to Reuters Health. “It is possible that regular sex a little earlier on, say from 36-37 weeks onward might have a different effect.”

There are good biological explanations for the widespread belief that having sex can bring on labor, Tan and colleagues note in the medical journal Obstetrics and Gynecology. For example, semen contains the hormone prostaglandin, which is used to induce labor, while orgasm can bring on contractions in the uterus. But there is virtually no scientific evidence that sexual intercourse near the end of pregnancy can jump-start labor.

To investigate, the investigators randomly assigned 210 women scheduled for labor induction to a group who were advised to have sex as much as possible before their induction, or to a control group who were told that it would be safe for them to have sex but that it wasn’t clear whether intercourse could bring on labor. Women received the advice 4.7 days before their induction was scheduled, on average.

Among the 108 women advised to have sex, 60 percent did so, compared to 40 percent of the 102 women in the control group. But among women who had intercourse, 56 percent went into labor on their own, compared to 52 percent of those who didn’t have sex, which was not a statistically significant difference.

“We speculate that women very close to spontaneous labor felt changes coming on that might reduce their libido,” for example pains and vaginal discharge, Tan told Reuters Health. “If correct, this would mean that those women who had sex were the least likely to go into spontaneous labor soon, so they would do less well in terms of spontaneous labor as outcome in the next few days. We did not have direct data to answer this question satisfactorily, however.”

“Based on the findings of our study, women scheduled for induction of labor at term should not be given advice to have sex for the purpose of promoting labor onset,” the investigators conclude.

SOURCE: Obstetrics & Gynecology, October 2007.