To determine the effects of aerobic fitness on pregnancy and the newborn, 20 primigravid subjects were studied throughout gestation and immediately postpartum. The subjects were classified as trained (T = 10) or untrained (UT = 10) based on the heart rate response to submaximal cycle ergometry testing done in each trimester. There was no difference between groups in the length of gestational period (T = 40.75; UT = 40.75 weeks) nor weight gained vs prepregnancy measures (T = 13.92; UT = 13.30 kg). The first stage of labor was extended in the UT, 13 h 58.8 min vs 11 h 18.0 min. UT had a longer second stage, 90.57 vs 70.0 min for T. Stage 3 was also prolonged in UT, 15.17 vs 7.43 min. In both groups medication was used equally. Two of the ten T females had Caesarean sections vs three of the ten in the UT group. The mean Apgar scores at 1 and 5 min were: T = 7.70, 9.20; UT = 7.90, 9.33, respectively. The birth weights of the T babies were marginally higher than the UT newborns (3733.00 vs 3679.97 g). All babies were healthy and without apparent abnormalities. There appears to be no positive or negative effects of maternal fitness on the newborn. The reduction in the active stage of labor in the T group may reflect their improved fitness levels.