Umbilical artery flow velocity waveforms in labour.

Doppler signals were recorded from the umbilical arteries in 103 women during labour. The pulsatility index (PI) did not alter with progress in labour. During uterine contractions the PI showed a change only if there was a deceleration in the fetal heart rate. Low birthweight (less than 10th centile) and delivery by caesarean section for fetal distress were significantly more frequent in patients with abnormal PI values (defined as greater than 2 SD above the mean in normal labours). Fetal scalp blood pH was measured in 24 patients. The PI was abnormal in only one case, but none of the 24 had a pH less than 7.20.