ABSTRACT
Monozygotic (MZ) twinning is a form of ‘vegetative’ reproduction whereby more than one individual results from a single zygote. The event has never been observed directly in humans. MZ twins at birth usually weigh somewhat less than twice the birth weight of singletons of corresponding gestational age, but the discrepancy is usually not great. Hence, a single fertilized egg is capable of producing much more somatic and placental mass than usual, but the mechanisms of overgrowth in MZ twinning are not fully understood. There may be extra mitotic cycles and/or reduced apoptosis, but it is a tribute to the plasticity of early embryogenesis that anatomically normal and wellgrown MZ twin, triplet, quadruplet and even quintuplet fetuses can be produced from one fertilized egg. Despite this, profound anatomic anomalies also result from MZ twinning (e.g. conjoined twinning, acardiac twins, twins discordant for regional malformations), and these outcomes have tainted the perception of MZ twinning with the reputation of being a teratologic or ‘freakish’ event whose mechanism is quite unknown and unknowable.
