Pathological analysis of trisomies 21, 18 and 13 was performed with materials from 1981 to 1989. The weight and microscopic findings in 48 placentas and the baby's weight were compared. Sixteen fetuses had trisomy 21, 25 fetuses had trisomy 18, and seven fetuses had trisomy 13. Thirty-nine placentas were trimmed and weighed. The weight ranged from 60g to 490g. There was a tendency for heavy placentas in trisomy 21, and a tendency for light placentas in trisomy 18 and trisomy 13, when compared to the standard weight. The baby weight correlated with the placental weight. The placentas of trisomic fetuses were complicated by immature villi in 35 (72.9%) of 48 cases. There were dysmature villi in 30 (62.5%) of 48 cases. Two different types of dysmature villi were found: one with many abnormal vessels; the other with a scarcity of vessels. Chorangiosis was revealed in 11 (22.9%) of 48 cases. An interesting vascular lesion has been identified in 12 (25.0%) of 48 cases, which was a villous hemorrhage without endovasculitis and villitis. These villous hemorrhages were associated with a low Apgar score. The human placenta in the presence of trisomy has been described as having immature or dysmature villi. We report two new pathological findings. One is that there were two different dysmature villi, one with many abnormal vessels, the other with a scarcity of vessels. The second is that villous hemorrhage without endovasculitis or villitis is one important factor related to a low Apgar score and fetal distress.