BOSTON, Sept. 29 -- Women who exercised four or more hours a week for years were 40% less likely to have a live birth after in vitro fertilization (IVF), according to a study of more than 2,200 patients.
Moreover, exercising four or more hours for one to nine years before attempting in vitro fertilization also doubled the risk of implantation failure, wrote Stephanie N. Morris, M.D., Harvard Medical School, and colleagues, in the October issue of Obstetrics and Gynecology.
"Interestingly, the most detrimental effect was observed in cardiovascular exercisers, who had a 30% lower chance of successful pregnancy after their first cycle of IVF," compared with women who didn't exercise, the investigators wrote.
By contrast, walking for one to three hours a week did not increase the risk of IVF failure, but women who walked for four or more hours a week "were 50% less likely to have a live birth compared with women who did not regularly exercise."
The authors concluded that while exercise has "many known health benefits, it does not seem to contribute to successful IVF outcomes."
But they also cautioned that their findings "are not strong enough to encourage women to abandon exercise and embrace a sedentary lifestyle." They said more studies are needed to confirm the findings.
The study recruited 2,232 women at three Boston-area IVF clinics. All women were prospectively enrolled, from 1994 to 2003, before undergoing their first cycle of in vitro fertilization. Primary endpoints were successful live birth, cycle cancellation, failed fertilization, implantation failure, and pregnancy loss.
Exercise history was obtained from a questionnaire administered before starting the IVF. Women were asked about medical history, environment, and lifestyle. They were about type, frequency and duration of exercise.
Of the 2,232 women enrolled, 1,368 said they exercised regularly. The mean duration of exercise was 11.3 years, for 10.8 months per years and 4.3 hours per week. Forty-two percent of women said that walking was their most common form of exercise, while 23% favored aerobic workouts, 15% said they jogged, and 8% said their regular exercise was indoor cycling or rowing.
Among the findings:
- Compared with women who didn't exercise, women who said they exercised regularly had a 20% lower likelihood of successful live birth, which was not significant (P=0.07).
- Regular exercisers had significantly increased risk of implantation failure (OR 1.3, CI 1.0-1.6; P=0.04).
- Women who exercised at least four hours a week for one to nine years, were almost three times more likely to have a cycle cancellation (OR 2.8, CI 1.5-5.3) and twice as likely to experience pregnancy loss (OR 2.0, CI 1.2-3-4) than women who did not exercise regularly.
The authors said that there are several possible explanations for the apparent relationship between exercise and IVF. For example, intense physical activities that emphasize leanness can disrupt the production of pulsatile gonandotropin releasing hormone from the hypothalamus, which results in a hypoestrogenic state.
"Overall, the incidence of luteal phase defects, oligomenorrhea, anovulation, and subsequent infertility are significantly higher in athletes than nonathletes; however, the mechanisms of the dysfunction may vary according to type of exercise," they wrote.
So, while IVF attempts to "manipulate the hypothalamic-pituitary-ovarian axis" exercise may be triggering a different manipulation, in effect an antagonistic one.
The authors noted several limitations of the study including the possibility of inaccurate reporting of exercise history, but any differential bias should be negated because the patients supplied the information before undergoing the first IVF cycle.
Also, it was difficult to quantify type, intensity, frequency and duration of exercise, so the authors were unable to study the differences between two two-hour exercise sessions per week versus four one-hour sessions or eight 30 minute sessions.
Finally the survey completion rate was 65%, and 70% is the desired completion rate for epidemiologic studies. But given the stress involved with IVF, the authors considered the rate was acceptable.
The study was funded by the National Institutes of Health.