Growth disturbance in the trisomic fetus is believed to be primarily fetal in origin. There has been only sparse description of placental pathology in the third trimester in these fetuses, and therefore the placental role in their growth and development remains unexplored. We performed quantitative morphometric analysis on the placentas of 18 fetuses with trisomy and ten normal control fetuses. Doppler umbilical artery analysis was performed on ten abnormal fetuses and all controls. The placentas of trisomic fetuses exhibited a significant reduction in small muscular artery count and small muscular artery/villus ratio. Abnormal Doppler waveforms correlated closely with reduced small muscular artery counts. Undervascularization and increased vascular resistance of the placenta of trisomic fetuses may contribute to diminished fetal growth. The placenta appears to be another fetal organ whose structure and function are affected adversely by abnormal karyotype.