For this study, researchers recruited 84 first-time mothers, who were of normal weight on average, and tracked any effects aerobic exercise might have on their insulin sensitivity and, ultimately, on their babies' birth weight. Exercise is known to lower the risk of insulin resistance —the precursor condition to diabetes. This is a good thing for healthy, non-pregnant adults, but some research indicates that insulin resistance may be helpful for proper fetal development. So it was unclear whether or not exercise would undermine a developing baby's food supply.
Researchers asked half of the women to exercise on a stationary bicycle for at least 40 minutes per session, up to five times each week, starting in the 20th week of pregnancy; the other half in the other group were not specifically asked to exercise. When the two groups and their babies were compared, the team found that women who bicycled regularly gave birth to babies who were on average about 5 ounces lighter than those born to the non-exercising mothers. In both groups, however, the babies were of healthy weight, and there was no difference in the mothers' weights.
In general, babies on the lower end of the normal weight range are considered healthier and less prone to developing diabetes and obesity than heavier ones, thus the researchers conclusion is that regular aerobic exercise for pregnant mom's is not only good for her baby, but may even help prevent childhood obesity later on in life.