To investigate the effects of exercise on the pregnant woman and her fetus, 20 pregnant women were studied during the second and third trimesters. Twelve of the women participated in an aerobic exercise program while eight women did not perform any regular exercise and served as a control group. On the basis of submaximal exercise test results, an 18% improvement and a 4% decline in absolute aerobic capacity (liters of oxygen per minute) were observed in the exercise and control groups, respectively. Functional aerobic capacity (milliliters of oxygen per kilogram per minute) rose 8% in the exercise group and declined 10% in the control group. A small but significant rise in fetal heart rate was measured during the exercise sessions. A comparison of pregnancy outcome of the two groups showed no differences in labor duration, Apgar scores, or fetal growth.