Zygotic-Splitting Rates after Single-Embryo Transfers in in Vitro Fertilization

Abstract

The mechanism of spontaneous monozygotic twinning is unknown. However, on the basis of observations first made 15 years ago, it is now generally accepted that all forms of assisted conception increase the rate of zygotic splitting.^1-3 The true incidence of zygotic splitting is unknown in a substantial proportion of twins, because same-sex dizygotic twins cannot be differentiated from dichorionic monozygotic twins by either antepartum ultrasonography⁴ or postpartum clinical examination.