ABSTRACT
Multifetal pregnancies are associated with various ante-and perinatal conditions that justifiably characterize them as high risk (see Chapters 49 and 50); however, it is not generally appreciated that the biological mechanisms underlying twinning are particularly complex and occur for the most part prior to fertilization. This chapter reviews those factors known or believed to influence the early embryonic division process that leads to monozygotic (MZ) twins.
