The average birth weight of babies born to mothers who exercised was 7.5 pounds, compared to 7.8 pounds for mothers who did not exercise, says a team of researchers from New Zealand and the U.S. Babies born weighing 8.8 pounds or more are defined as high birth weight.
At the two-week checkup, the babies of exercising moms averaged 8.1 pounds; the babies of sedentary moms averaged 8.6 pounds.
"We would suggest this study support the recommendations of at least 30 minutes of moderate exercise daily and probably more," says study co-author Paul Hofman, MD, a researcher at the University of Auckland in New Zealand. The study is published in the Journal of Clinical Endocrinology & Metabolism.
The new research follows a recent study finding that three out of four pregnant women in the U.S. do not get enough exercise.
Exercise During Pregnancy: The Study
Hofman and his colleagues assigned 84 women pregnant with their first child to an exercise group or a control group. Women in the exercise group rode stationary bikes at home at a moderate intensity for 40 minutes, 5 times a week maximum, beginning at 20 weeks into the pregnancy and continuing until about week 36.
Women in the control group were instructed to continue their normal daily activities during the same time periods.
Women in both groups had, on average, a healthy body weight before pregnancy and were similar in other regards such as age and ethnicity.
Exercise During Pregnancy: Results
The babies born to the exercisers had a lower body weight and a lower body mass index or BMI.
There were no differences in the length of the babies, on average, between exercisers and non-exercisers. The exercise didn't affect the length of the pregnancy, either, or the mothers’ weight.
Exactly why the exercising mothers produced smaller babies isn't known, says study co-author Chris Baldi, PhD, an assistant professor of biological sciences at Northern Arizona University, Flagstaff.
Researchers initially believed the exercising mothers would become less insulin-resistant. Typically, Baldi says, a woman becomes more insulin-resistant during pregnancy, a normal occurrence within certain boundaries. "It has a lot to do with hormonal changes," he says. If a woman becomes too insulin-resistant, gestational diabetes can occur.
"It did not come out as we suspected," Baldi tells WebMD, with no substantial differences found in insulin resistance between the two groups.
The difference, says Baldi and Hofman, may be because of other factors, such as differences found in certain growth factors. Another possible explanation, Hofman says, is slightly reduced blood flow that occurs to the placenta during exercise and for several hours afterward.
Exercise During Pregnancy: Other Input
The finding of lower birth weights of infants of exercising mothers is not surprising to Linda May, PhD, an associate professor of anatomy at the Kansas City University of Medicine and Biosciences in Kansas City, Mo., who has also researched the topic.
The finding that there was no change in insulin sensitivity between groups was surprising, she tells WebMD.
Her research has found that during pregnancy, more exercise [within recommended amounts for an individual] yields more benefits.
More study is needed on high-risk pregnant women, Hofman and May say. The women in the study, exercisers and non-exercisers, were not high risk.
For most healthy pregnant women, Hofman says, the research seems to support exercising at least 30 minutes a day moderately, and probably more. Some studies, he says, have demonstrated up to 60 minutes daily is generally safe.