NEW YORK (Reuters Health) - Regular exercise during pregnancy can help women and their newborns, but the particulars of this benefit depend on the intensity and timing of the exercise regimen, study findings suggest.

Previous studies have shown that, in general, women who exercise throughout their pregnancies have larger placentas than their more sedentary peers but that a more intense exercise regimen may result in babies who weigh less at birth.

The volume of the placenta is a general marker of the structure's ability to transport oxygen and nutrients to the fetus, Dr. James F. Clapp III, lead author of the current study, explained in an interview with Reuters Health.

In their study, Clapp and colleagues found that women who exercised at the same rate throughout their pregnancy, or boosted the intensity of their exercise regimen later in pregnancy, gave birth to infants who were lighter and had less body fat than women who slowed down in the final trimester.

However, none of the babies born to women who exercised moderately or intensely through the ninth month were considered low birth weight, according to the report published in a recent issue of the American Journal of Obstetrics and Gynecology. Low birth weight is associated with developmental problems in childhood and may be linked to heart disease and other health problems later in life.

The findings indicate "that regular weight-bearing exercise is beneficial for both mother and baby and that the amount of benefit varies with the timing and amount of exercise," Clapp told Reuters Health. "If the woman is healthy and the pregnancy normal, regular exercise can do nothing but improve the situation."

Clapp and colleagues from Case Western Reserve University in Cleveland, Ohio assigned 75 women who exercised regularly to one of three aerobic exercise programs during their eighth week of pregnancy. Women worked out on a treadmill, took step aerobics or used a stair-stepping machine.

Women in the "Lo-Hi" group did 20 minutes of aerobic exercise 5 days a week and increased the duration to 60 minutes by the 24th week of pregnancy until their delivery. Women in the "Hi-Lo" group exercised for 60 minutes 5 days a week through week 20 and then decreased the duration to 20 minutes from week 24 until the end of the pregnancy.

In the third group, women exercised for 40 minutes 5 days a week throughout their pregnancy.

Infants of women who exercised intensely in the middle or late stages of pregnancy were significantly lighter and had less body fat than infants born to the moderate exercisers, although none of the infants were small enough to be at risk for medical or developmental problems.

Women who exercised vigorously early on and then reduced the intensity of their program as they approached their due dates weighed more and had heavier placentas than women in the other groups. While a heavier placenta can be protective in some cases, the heavy exercisers did not have dangerously light placentas, Clapp said.

He suggests that future studies investigate the use of exercise as a way to prevent complications in certain individuals, including women at risk of delivering premature and low birth weight babies, as well as the effects of different types of workouts such as scuba diving and spinning.

SOURCE: American Journal of Obstetrics and Gynecology 2002;186:142-147.

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