The influence of aerobic swin conditioning on levels of maternal physical fitness and the influence of each swim bout on selected maternal and fetal circulatory parameters were investigated. Thirteen women who met criteria for a low-risk homogeneous sample were randomly assigned to either an experimental (n = 7) or a control (n = 6) group. Experimental subjects participated in ten weeks of a swim conditioning program, while control subjects maintained their normal activities (to exclude aerobic conditioning). Fitness levels for all subjects were determined from respiratory gas analysis during graded exercise testing on a treadmill prior to and following the swim conditioning program. Testing indicated that experimental subjects were able to maintain their initial fitness levels over the twelve weeks of increasing gestation, while control subjects were not able to do so. These findings suggest the effectiveness of the swim conditioning program in countering a potential decrease in fitness levels. Importantly, maternal blood pressure, pulse, and fetal heart rate response remained within clinically normal limits during swim conditioning for experimental subjects, and during treadmill testing for all subjects.