Moderate exercise is healthy for pregnant women and their babies, but exercising too much or too little could raise the risk of bearing a low birth weight baby.

The investigators found that women who exercised strenuously five or more times a week during the last trimester of their pregnancy had four times the risk of having a low weight baby.

Women who exercised fewer than three times a week were twice as likely to have a low birth weight baby.

Low birth weight babies are believed to be more likely to have subsequent health problems.

Pregnant women who exercised three or four times a week seemed to have the best chance of having a healthy weight baby.

Pregnancy is not the time to exercise excessively, but it is also not the time to be sedentary. Moderate exercise three to four times a week is perfectly safe -- and may even be beneficial.

The investigators found that doing strenuous exercise, such as aerobics classes, more than four times a week increased substantially the risk of having a low birth weight baby.

Other researchers have theorized that too much exercise could possibly draw blood towards the mother's exercising muscles and away from the developing fetus, depriving it of oxygen and nutrients.

The researchers found no evidence that a woman's level of pre-pregnancy fitness had any affect on the association between exercise and birth weight.

The researchers had expected that women who were fit before pregnancy might be able to tolerate a high level of exercise without affecting their baby's weight, but this was not the case. And women who had not exercised did not increase their risk by starting to exercise modestly while pregnant.

Staying sedentary did increase their risk of delivering a low birth weight baby.

American Journal of Obstetrics and Gynecology February 2001;184:403-408