The biology of the twinning process: how placentation influences outcome.

Placentation of twins has important bearing on the fetal development and neonatal outcome. Because of limitation in space of the endometrial cavity, twin placentas frequently collide during their expansion. This may restrict their growth and also lead to a somewhat increased rate of placenta previa. Moreover, abnormal cord insertions such as marginal and velamentous cords are much more frequent in multiple gestation. Absence of one umbilical artery is also much more frequent in twins. The best prognosis is had when two independent placentas develop, the dichorionic separate organs. Fusion of two separate placentas does not lead to vascular anastomoses in human twins, whereas it often does in other species (marmoset, cattle). Blood vessel fusion occurs only in monochorionic twins, all of which are "identical" (monozygotic). Large anastomoses (usually artery-to-artery) allow blood to shift rapidly from one to the other. Thus, when one twins dies, the survivor may quickly exsanguinate into the dead twin, leading to hypotension and occasionally to cerebral palsy. Arteriovenous anastomoses are the basis for the twin transfusion syndrome. This results in severe prematurity and gross discordance of all sorts of physical parameters. Knowing this type of vascular shunts helped develop the prenatal laser obliteration with salvage of the twins. The worst prognosis occurs with the rarest type of placentation in twin, in which both reside in the same cavity. This is the monoamnionic-monochorionic twin placentation. By moving about, the twins often entangle their umbilical cords and some 40% to 50% may die in utero.