Two thirds of twin pregnancies are dizygotic, and all these twin pairs are dichorionic. Among monozygotic twins, 70% have monochorionic and 30% have dichorionic placentation. All monochorionic twins and 10% of dichorionic twins are monozygotic. Observations that fetal outcomes are worse for monochorionic than for dichorionic twins have been ascribed to complications caused by placental vascular anastomoses, but whether absolute outcomes are related to monozygosity or monochorionicity remains uncertain. This prospective study of women having twin pregnancies in the years 1998-2000 was intended to relate fetal outcomes to zygosity and chorionicity. The study compared two kinds of dichorionic twin pregnancies resulting in live births: 92 dizygotic and thus dichorionic pregnancies and 15 monozygotic, dichorionic pregnancies. Zygosity was determined using microsatellite markers amplified by the polymerase chain reaction technique, and the placentas were examined microscopically to determine chorionic type. In all 51 dizygotic twin pregnancies, in which the infants were of the same sex, the placenta was classified as dichorionic. All 41 pairs of different-sex twins were dizygotic. In seven of 39 pairs of monozygotic monochorionic twins, either both twins were not born alive or they were delivered before 24 weeks gestation. Monochorionic pregnancies had significantly lower gestational ages at delivery as well as significantly lower birth weights. Cases with birth weight discordance of more than 25% were more numerous in monochorionic pregnancies than in the other groups. All 10 fetal losses were in the monochorionic group; six of them were related to twin-twin transfusion. No significant differences in any parameters were found between the monozygotic dichorionic and dizygotic groups. These findings demonstrate that, in twin pregnancies, fetal outcomes are related to chorionicity, not zygosity.