SummaryThe velocity of blood flow was estimated by Doppler examination in three sites of the fetal circulation repeatedly throughout the day to evaluate the diurnal variations in these measurements. The intracardiac, umbilical arterial and uterine arterial waveforms were recorded in 24 normal pregnancies, ranging from 18 to 30 weeks gestational age. Studies were performed at 10 a.m., 1–2 p.m. and 4–5 p.m.Analysis of variance showed no significant change in combined cardiac output, or in uterine or umbilical blood flow velocities in this gestational age range, between the three times of the day.