Getting Fit—or Getting Pregnant?

Exercise is usually a key part of what the doctor orders. However, a recent study published in the journal Obstetrics and Gynecology suggests there’s at least one instance when limiting exercise may be the proper prescription—trying to conceive via in vitro fertilization (IVF).

The study, conducted by Harvard Medical School researchers, followed more than 2,200 women through their first IVF cycle. Compared to their sedentary counterparts, women who reported walking (or engaging in cardiovascular exercise like jogging) at least four hours per week for one to nine years prior to undergoing IVF were 50 percent less likely to get pregnant. One to three hours per week of cardiovascular exercise—more intense than everyday walking—still reduced a woman's chance of giving birth by 30%.

“Although exercise has known benefits, it does not seem to contribute to successful IVF outcomes,” the researchers concluded, though they cautioned that more studies are needed before making definitive clinical recommendations.

One fertility expert who isn’t inclined to wait for more studies is Alice Domar, a health psychologist who teaches at the Harvard Medical School and also directs the Mind/Body Center for Women’s Health in the Boston area.

“We’ve known for a long time that excessive exercise in animals can adversely impact their fertility,” Domar says. So for more than 15 years, she’s been counseling women with fertility issues to moderate their level of exercise.

“I’ve had a lot of patients cut back on exercise,” says Domar. “And a lot of them have gotten pregnant right away. If I had not seen clinical success, I would’ve stopped recommending this a long time ago. But I’ve had too many patients get pregnant.”

Domar believes there’s a reasonable evolutionary explanation for this seemingly surprising phenomenon. “The only reason that we can run is to escape from danger,” Domar explains. “So while a woman’s reasons for running may have nothing to do with escaping a saber-toothed tiger, her body doesn’t know that.”

“Her body doesn’t know that she’s running so she can eat crème brûlée tonight. Instead, it thinks she’s living in a really dangerous environment and surviving because she’s fast. So is her body going to let her get pregnant and fat and slow?”

What’s Wrong With Exercise?

Exercise decreased ultimate birth rates by negatively impacting three of four key stages of the IVF process: increased cancelled cycles because there were no eggs to harvest, implantation failure, and miscarriage after embryo transfer. The only stage not affected was the capability of harvested eggs to be fertilized outside of the women’s bodies.

There are several ways exercise can impact fertility. It’s been known for some time that women who lack sufficient body mass (think lean gymnasts and long-distance runners) can cease to ovulate and menstruate. It's also known that intense physical exercise can disrupt the delicate and complex hormonal and biochemical balance that’s needed to achieve and sustain a pregnancy.

One important note: The study looked at the impact of exercise prior to IVF embryo transfer. After transfer, most clinics advise women to forgo exercise, heavy lifting and other strenuous activities.