Exercise builds big babies.

Many physically fit women can exercise vigorously throughout pregnancy, a new study suggests. Not only does working out retain fitness, but it also helps produce bigger babies, the research shows. And that can be good news, because larger babies tend to weather physical adversities better.

Maureen C. Hatch and her co-workers at Columbia University in New York City observe that most studies of prenatal exercise have focused on occupational activities, which often require unnatural or repetitive motions that can strain the body, The Columbia team instead looked at recreational exercise, which tends to be rhythmic and build fitness.

In the just-released May 15 AMERICAN JOURNAL OF EPIDEMIOLOGY, they report that women who regularly burned up to 1,000 calories per week in recreational pursuits delivered babies weighing about 5 percent more than babies of inactive moms. Those who burned 2,000 calories a week gave birth to infants nearly 10 percent heavier. A total of 462 suburban and rural women participated in the study,

The American College of Obstetricians and Gynecologists recommends that pregnant women limit the type, intensity, and duration of exercise: how much depends upon a woman's fitness prior to pregnancy and the presence of obstetrical risk factors. Hatch says her data suggest that these guidelines may be overly stringent for fit women at low risk of problem pregnancies.