Amniotic band syndrome in monozygotic twins: prenatal diagnosis and pathogenesis.

Amniotic band syndrome is a well-described disorder lacking a precise definition or a scientifically validated hypothesis of pathogenesis. The widely accepted "exogenous" theory suggests that early amniotic rupture leads to the formation of pathologic amniotic strands, which then induce nonanatomic fetal abnormalities. This paradigm appears to be challenged by observations that amniotic band syndrome occurs in monozygotic twin gestations. The exclusive development of amniotic band syndrome in monozygotic versus dizygotic twin gestations, the description of early amniotic rupture in one sac of a dizygotic twin gestation without subsequent fetal abnormalities, and the paradoxical observations of discordance in monoamniotic and concordance in diamniotic twin gestations, fail to support an "exogenous" etiology for amniotic band syndrome.