Monozygotic twinning in the human is associated with the zona pellucida architecture

Six cases of identical twin pregnancies which occurred in 2163 cycles of in-vitro fertilization during a 3 year period are reported. Monozygosity was confirmed when the number of fetuses exceeded the number of embryos replaced (n = 4) or when two concepti were observed in a single amniotic sac (n = 2). Each of the reported pregnancies resulted from replacement of embryos either with naturally thin zonae pellucidae or embryos whose zonae had been breached during micromanipulation for assisted fertilization (subzonal sperm insertion) or assisted hatching. That such cases exclusively gave rise to monozygosity suggests a link between the physical state of the zona pellucida, hatching, and generation of identical twins.