Twenty-one nulliparous women in the last trimester of pregnancy gave their informed verbal consent to undergo an acute exercise test. Fetal umbilical artery velocimetry waveforms were measured to monitor the effect of exercise on fetal umbilical artery blood flow. The mean gestational age was 32 weeks (range 28-37 weeks). The mean exercise intensity was 71% of submaximal exercise (range 67-74%). Maternal heart rate rose significantly (P less than 0.001) from a mean of 83 to 138 beats per min. Following moderate exercise, the systolic over the diastolic ratio (S/D) decreased significantly (P less than 0.001) after 2 and 20 min. This indicates a decrease in umbilical blood flow resistance and an improved placental circulation following exercise in healthy women. We conclude that maternal exercise does not appear to be harmful to the mother or her fetus at this level of exercise intensity in healthy individuals.