Animal studies have suggested a detrimental effect of exercise on uterine blood flow, but it is unclear whether the same responses occur in human pregnancy. Thirty-four women in the third trimester of singleton pregnancies underwent a bicycle exercise test during which a pulsed Doppler method was used to assess the response of the uteroplacental circulation. Twelve pregnancies were uncomplicated and the other 22 were complicated by hypertension, small for gestational age (SGA) fetus, or both. Exercise appeared to increase the pulsatility of the uteroplacental Doppler waveform in all cases. The changes in the waveforms were more exaggerated in the complicated pregnancies, particularly when the resting waveform had been abnormal. These changes indicate an increase in uteroplacental vascular resistance with exercise, suggesting a deleterious effect of physical exertion in the third trimester, particularly in the presence of hypertension or SGA fetus.