Monoamniotic twin pregnancy: a review of contemporary practice.

Monoamniotic twins represent an uncommon manifestation of the monozygotic twinning process (1% of monozygotic twins). They are the result of ovum division beyond 8 days postconception and are characterised by a single amnion and a single yolk sac. There may be two or one (conjoined twins) embryos present. This form of twinning is notable for an increase in perinatal loss (10-15% in contemporary series of prenatally recognised cases). Congenital anomalies, umbilical cord entanglement/accidents, preterm birth and intrauterine growth restriction primarily account for this increased perinatal mortality rate. Diagnosis is most typically by ultrasound, with the inability to distinguish a dividing membrane between the fetuses the most typical feature. There are diagnostic pitfalls (oligohydramnios in one twin with closely adherent membrane) and accuracy in diagnosis is central to appropriate pregnancy management. Obstetric management protocols are based on several retrospective case series, and there is a paucity of large well-controlled studies to provide guidance in the management of monoamniotic twin pregnancies. The prevention of antepartum death from umbilical cord entanglement is one of the central management issues; however, the available techniques of antepartum surveillance have a variable performance in predicting this event.