Sixteen primigravid patients were studied on four occasions (once in each trimester and again at 4 weeks' post partum) for predicted VO2 max (Canadian Home Fitness Test), body weight, and percentage of body fat (skin folds) and caloric intake (3 day food diary). The VO2 max declined significantly from first to second to third trimester and rose post partum to the second-trimester level. Energy intake was constant but body weight and fat rose progressively until after birth. When subjects were divided into "fit" (n = 8) and "unfit" (n = 8) groups, the fit group had significantly greater VO2 max, lower heart rate and diastolic pressure, and less body fat. These differences were not present at third trimester but were present again in the postpartum data. There were no differences in delivery or neonatal data.