Umbilical artery blood velocity waveforms were compared with the morphological aspects of the vascular bed of 30 placentas using quantitative methods. An abnormal pulsatility index was found to be highly correlated with a lower number of arteries in the tertiary stem villi as well as a lower placental weight and birth weight. The reduction in the number of arteries seems to be due to an early developmental arrest of placental angiogenesis and is partly compensated for by increased formation of capillaries in the terminal villi, since the number of capillaries is independent of the number of arteries.