What about baby?
That’s all fine for Mom — but what about the effects of exercise on baby? While there is much that medical experts still don’t know, because of ethical concerns about women participating in research that might harm their unborn babies, there’s reassuring news on that front, too.
In the largest study to date, published in the American Journal of Epidemiology in early 1996, researchers from the Missouri Department of Health surveyed nearly 3,000 women who recently had given birth, asking about their general health, work activities and exercise regimen. They found that the amount of exercise or daily activities an expectant mother engaged in did not increase the risk of fetal death or a low birth weight baby. Of their subjects, about 23 percent had exercised three or more times a week in their first trimester, 19 percent in their second trimester and 16 percent in their third trimester. More than half the women walked for a workout, while the rest did something else.
Earlier studies already had eased various concerns, such as the worry that exercise would cause a woman to suffer from hyperthermia (excessive overheating), which can trigger fetal distress and premature labor. In fact, women who exercise during pregnancy have been shown to dissipate heat more efficiently, because their well-trained bodies start sweating at a lower body temperature.
Best of all, a mother’s exercise may have benefits for her baby down the line, according to a new study by James Clapp. He compared the offspring of 20 women who did moderate aerobic exercise (greater than 55 percent of maximum capacity) at least three times a week for 30 minutes or more during pregnancy to the offspring of 20 women who only walked normally while pregnant. At age 5, the children of the exercisers had less body fat and higher scores on tests of general intelligence and oral language skills.
Only four years ago, the American College of Obstetricians and Gynecologists was advising pregnant women not to exercise longer than 15 minutes at a time, and to keep their heart rate below 140 beats per minute. Among other fears, doctors believed that strenuous exercise could trigger early labor or divert blood flow from the fetus. In 1994, though, after a review of the research, ACOG dropped the heart-rate and duration restrictions and affirmed that women could continue to do moderate exercise throughout pregnancy — with their doctor’s blessing, of course, and barring certain medical complications.
That doesn’t mean that health debates don’t still remain. Some fitness experts still feel that exercise puts women at greater risk of joint injury, due to the effects of the pregnancy hormone relaxin, which loosens the cartilage in the pelvis joints so the baby’s head can pass through the birth canal. Scientists have not been able to show that relaxin affects other joints in a pregnant women’s body, however. While pregnant women may be more susceptible to joint injuries, relaxin is not the culprit: It’s women’s heftier size in late pregnancy, says Michelle Mottola, Ph.D., director of the R. Samuel McLaughlin Foundation Exercise and Pregnancy Lab at the University of Western Ontario in London, Ontario, Canada.
Much more study of exercise and pregnancy remains to be done. “We’re still lacking good information for the highly trained woman,” says James Pivarnik, Ph.D., a professor of exercise science and osteopathic surgical specialties at Michigan State University. Experts also are largely unsure about how much exercise pregnant women should do, but at least part of the answer lies with the individual’s conditioning and motivation.