Sesquizygotic twinning: a unique twinning type rather than mechanism

Sesquizygotic twinning (SZT) is one of the rarest events that can occur in multiple pregnancies. It has only been reported twice to date. Studies indicate that sesquizygotic twins result from the splitting of a chimeric embryo at an early stage. Theories and studies suggest that SZT represents a third unique twinning mechanism that is different from dizygotic and monozygotic twinning; however, the unique features of SZT have nothing to do with the twinning mechanism. Instead, it resembles more of an atypical monozygotic twinning form according to the twinning mechanism.