Zygosity testing of all multiple births allowed the identification of a subgroup of 42 monozygotic twin pairs who have dichorionic placentas, fused and separate. Perinatal outcomes of this group were compared with 110 pairs of monochorionic monozygotic twins and 148 pairs of dizygotic twins. Dichorionic monozygotic twins had the lowest incidence of preterm birth, perinatal mortality, and birth weight discordance. There was an excess of like-sexed over unlike-sexed pairs among the dizygotic twins.