Placental microvascular anatomy was correlated with antenatal assessment of the umbilical circulation in 106 patients to further validate the measurement of the A/B ratio (the ratio of peak systolic to least diastolic flow velocity) of the umbilical artery flow velocity time waveforms as an index of blood flow resistance. Three groups of patients were studied: a normal group of 38 uncomplicated pregnancies, a control group of 33 potentially 'at risk' pregnancies with a normal A/B ratio matched by risk factors and gestation with the third group of 35 pregnancies with a high A/B ratio. Placental arterial resistance was quantitated by counting the number of small muscular arteries (less than 90 micron diameter) in the tertiary stem villi in a standard microscopic field (mean 18.5 fields/placenta). The modal small arterial vessel count was shown to be significantly less in the group with a high fetal risk and a high A/B ratio (1-2 arteries/field) than in both the normal and control groups (7-8 arteries/field). The tertiary villus count did not vary between groups. Antenatal studies of umbilical artery flow velocity waveforms with Doppler ultrasound identify a specific microvascular lesion in the placenta characterized by obliteration of small muscular arteries in the tertiary stem villi.