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Exercise During Pregnancy for Smaller Baby

Moderate Exercise During Pregnancy Lowers Baby's Birth Weight, May Reduce Baby's Risk of Obesity
WebMD Health News
Reviewed by Laura J. Martin, MD

April 5, 2010 -- Women who exercise moderately during pregnancy give birth to somewhat smaller babies, which may reduce the infants' obesity risk later, according to a new study.

The average birth weight of babies born to exercising mothers was lower but still very healthy.

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.

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