ABSTRACT
Twins can either be dizygotic (70%) or monozygotic (30%). Dizygotic twins result from the fertilization of two eggs, and there is a separate placenta and amniotic sac for each twin. Dizygotic twin pregnancies are, therefore, by definition dichorionic diamniotic and the few reported exceptions of dizygotic monochorionic twins due to fusion of two separate blastocysts only confirm this rule (1,2). In contrast, monozygotic twins arise from the fertilization of a single egg with subsequent splitting of the zygote. The longer the time period between fertilization and splitting, the more structures the twins have in common. In about 75% of monozygotic twins, splitting takes place after the third day postfertilization, resulting in a single placenta for both fetuses (monochorionic). The majority of monochorionic twins have separate amniotic sacs (diamniotic). Rarely, however, splitting occurs beyond the ninth day and these twins also share a single sac (monoamniotic), whereas splitting after the 12th day will result in conjoined twins. About 25% of monozygotic twins will be dichorionic diamniotic, when splitting occurs before the third day after fertilization.
