Uterine artery blood velocities during contractions in pregnancy and labour related to intrauterine pressure

Doppler recordings from branches of the uterine artery were made simultaneously with intrauterine pressure measurements in eight women in labour. Blood velocities fell and pulsatility index (PI) increased during all 22 contractions studied. Velocities were reduced almost linearly by up to 60% when pressure increased from 0 to 50 mmHg. To study the effect of uterine contractions before labour on uterine blood flow, simultaneous Doppler recordings from branches of the uterine artery and external tocodynamometry were made in seven women during the last trimester. Velocities fell during 18 of the 20 contractions studied (median 35%, range 19–71%). Extrapolating from labour contractions, this corresponds to a pressure increase of 30–40 mmHg. Fetal heart rate (FHR) recordings were normal during all contractions.