OBJECTIVE To compare umbilical arterial and venous Doppler velocity waveforms during labor in fetuses with normal heart rate tracings and fetuses with late decelerations. METHODS During labor, umbilical arterial and venous Doppler flow velocity waveforms were obtained between and during contractions in 20 fetuses (ten with normal heart rate tracings and ten with late decelerations). The umbilical arterial systolic-diastolic (S/D) velocity ratios were compared, and umbilical venous velocities were examined for the presence or absence of venous pulsations. RESULTS Fetuses with late decelerations had a significant increase in the umbilical arterial S/D ratio between contractions compared to normal fetuses (2.43 +/- 0.74 versus 1.56 +/- 0.27, P = .003). There was no significant difference in S/D ratios obtained between contractions compared with those obtained during contractions in either the normal or late deceleration group. Umbilical venous pulsations were significantly more common (90 versus 0%, P < .00001) during contractions in fetuses with late decelerations; umbilical venous pulsations were not seen in these fetuses between contractions and were not present at any time in fetuses with normal heart rate tracings. CONCLUSIONS Increased umbilical arterial velocity ratios and the presence of umbilical venous pulsations during contractions in fetuses with late decelerations suggest that the cardiovascular pathophysiology of late decelerations involves changes in placental resistance as well as cardiac performance. The presence of umbilical venous pulsations during late decelerations is consistent with the hypoxic fetal cardiovascular changes (hypertension and myocardial depression) previously reported from animal studies.