The placentas of 14 growth-retarded fetuses with abnormal umbilical artery velocimetry and 15 appropriately grown fetuses with normal waveforms were collected immediately after delivery and analyzed in a blinded fashion. A large percentage of the arterial vessels in the placentas from the former group showed abnormal changes in the vessel wall. The percentage of abnormal arterial vessels in all placentas correlated significantly with the resistance index. This study provides an anatomical basis for the elevated resistance to blood flow in placentas from growth-retarded fetuses.