Using pulsed Doppler and B-mode ultrasonic techniques, umbilical venous flow has been measured for the first time under essentially normal physiological conditions. In normal pregnancies, the flow per unit fetal weight remains essentially constant at 110-120 ml/min/kg for most of the pregnancy. In pregnancies with complications, however, abnormally low or high flow values are frequently observed. Low flow values correlate strongly with retarded fetal growth, and with increased incidences of antenatal hypoxia, neonatal morbidity and neonatal death. In some circumstances high flow values suggest the presence of a compensatory mechanism. The results reported here suggest that umbilical flow can be used to separate all fetuses, whether growth retarded or not, into "low risk" and "high risk" groups with better sensitivity and accuracy than existing methods. In addition, low flow values have been measured an average of one week before growth retardation or fetal hypoxia were indicated by the conventional methods. A possible strategy for the diagnostic use of umbilical flow measurements is outlined.