The flow velocity waveform profile in the umbilical artery was measured in 50 pairs of twins during the last week before birth. A significant association was found between high resistance indicies and occurrence of late fetal heart rate decelerations; the higher the resistance, the more frequent the decelerations. Further, the connection between high resistance in the circulation of the umbilical artery and low birth weight was confirmed. Flow velocity measurements in the umbilical artery seem to be a valuable tool in identifying twin fetuses suffering from placental insufficiency.