While doctors now encourage women to exercise through pregnancy, fertility specialists have long warned women with infertility issues to cut back on vigorous exercise (like training for a marathon) because it might overtax the body and prevent ovulation. But that may not be the case for women who are overweight, according to a new study from Boston University School of Public Health that was published Thursday in the journal Fertility and Sterility.

The researchers compared self-reported exercise habits among more than 3,600 Danish women who were attempting to get pregnant and found that women at a healthy weight who reported exercising vigorously - running, fast cycling, swimming, aerobics classes - for five or more hours per week had a 42 percent reduced likelihood of becoming pregnant in any given month compared to those who did light exercise or none at all.

That effect of vigorous exercise, however, wasn’t seen in overweight women who had a body mass index of 25 or more (150 pounds or greater for a 5’5” woman). If anything, vigorous exercise appeared to slightly enhance fertility in these women, reducing the number of months it took to get pregnant, but that effect wasn’t big enough to be statistically significant.

Althought the study couldn’t prove that strenous activity actually delayed conception, the researchers controlled for other factors that could have explained the delay like alcohol and caffeine intake, intercourse frequency, smoking habits, and previous childbirths.

“The take home message is that moderate physical activity appears to enhance fertility among all groups of women,” said study author Lauren Wise, an associate professor of epidemiology at Boston University School of Public Health. “Women at a healthy weight who exercise vigorously may want to switch to lighter activity if they’re having trouble getting pregnant.”

Biological reasons to explain why intense exercise would impair fertility in thin women and not their overweight counterparts remain unknown, but Wise speculated that it could have something to do with hormones. Exercise helps control the release of estrogen, which is often abnormally high in overweight women, contributing to infertility. In thin women, excess exercise may increase level of stress hormones, which are known to impair fertility.

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By Deborah Kotz, Globe Staff
Deborah Kotz can be reached at .(JavaScript must be enabled to view this email address). Follow her on Twitter @debkotz2.