Too Much Exercise May Delay Pregnancy in Normal-Weight Women: Study

March 15, 2012

Exercise is a plus for women trying to become pregnant, but overdoing workouts might make it harder to conceive — unless a woman is overweight — according to a new study led by BUSPH researchers working with Danish colleagues.

The study, published March 15 in the journal Fertility and Sterility, found evidence for a relationship between increasing levels of vigorous exercise and longer times to conception in healthy, normal-weight women who are planning to become pregnant. In overweight and obese women planning pregnancy, however, vigorous physical activity did not result in delays.

While moderate physical activity was associated with a small increase in fertility rates among all women, that outcome changed with “higher levels of vigorous exercise” among normal-weight women, said study author Lauren Wise, associate professor of epidemiology at BUSPH.

She said the findings indicate that physical activity of any type might improve fertility among heavier women, but that their normal-weight counterparts who want to improve their pregnancy odds should opt for low-key workouts, such as brisk walking or gardening. In other words, she said, marathon runners who want to conceive might want to scale back their workouts.

Usually a risk factor for health problems, being overweight or obese didn’t hinder fertility in heavy women who logged vigorous workouts — such as running, fast cycling and aerobics.

Wise noted that in other research, being overweight or obese has been associated with lower fertility rates compared to normal-weight women. Obesity also has been associated with menstrual cycle disturbances. She said the study suggests that increased physical activity might reverse the harmful effects of obesity and improve overweight women’s overall fertility rates.

The exact mechanisms for why physical activity might enhance fertility in overweight women remain unclear, though, and the researchers didn’t delve into them, Wise said.

Other studies of competitive female athletes suggest that intense workouts disturb women’s monthly menstrual cycles and lead to a lack of ovulation and even the absence of menstrual periods, among other problems, Wise said. But, she added, high-intensity exercise might also impair implantation when a fertilized egg attaches to the wall of the uterus.

The researchers of the observational study recruited and administered questionnaires to 3,628 women who ranged in age from 18 to 40. Participants had to be in stable relationships with male partners and planning to become pregnant, but not involved in any fertility treatments.

The researchers collected information on height, weight, reproductive and medical history, plus lifestyle and behavioral details, and then sent out follow-up questionnaires every two months for 12 months, or until a woman became pregnant.

The participants were categorized by their exercise exposure, and the results were further stratified by BMI (body mass index) to see whether the effects of physical activity on time to pregnancy differed, according to body weight.

Activities such as running, fast cycling, aerobics, gymnastics, and swimming were characterized as vigorous. Brisk walking, leisurely cycling, golfing and gardening were considered moderate. Assigning values– called total metabolic equivalents (METs)– to the activities, the researchers quantified the amount of moderate and vigorous physical activity each participant engaged in.

Linda Giudice, president-elect of the American Society for Reproductive Medicine, said the study was particularly interesting “because its participants were recruited from the general population, not from infertility patients.

“It points out the benefits of moderate exercise to all women who are planning a pregnancy and suggests that women might reduce their time to pregnancy by modulating their exercise programs,” she said.

Besides Wise, a researcher with the Slone Epidemiology Center, co-authors included: Kenneth J. Rothman, professor of epidemiology; Elizabeth Hatch, associate professor of epidemiology; Henrik Toft Sørensen, adjunct professor of epidemiology; and Ellen M. Mikkelsen and Anders H. Riis of the Department of Clinical Epidemiology, Aarhus University Hospital, Denmark.

submitted by: Lisa Chedekel

chedekel@bu.edu