Yolk-sac number in monoamniotic twins.

Background: It has been accepted practice to use yolk-sac number to predict amnionicity in twin pairs. Two yolk sacs in a single chorionic sac were thought to confirm diamniotic pregnancy, and a single yolk sac with two fetal poles was thought to indicate monoamniotic twins. Thus, basic ultrasonography could confirm amnionicity early in pregnancy and avoid invasive testing or the more difficult tasks of demonstrating the absence of a dividing membrane or presence of entwined cords later in the pregnancy.

Methods: We present a case of monoamniotic twin gestation in which early ultrasonography confirmed the presence of two yolk sacs. Doppler showed entwined umbilical cords. After elective cesarean delivery, the twins did well in the neonatal intensive care unit and continue to thrive.

Conclusions: Yolk-sac number does not predict amnionicity. The diagnosis of monoamniotic twins requires demonstration of entwined cords or an invasive procedure.