Antepartum rupture of diamniotic membranes separating monozygotic twins. A case report.

The partitioning membrane of a monochorionic, diamniotic twin gestation was visualized by ultrasound at weeks 18 and 22. The pregnancy progressed without difficulty to week 37. Delivery by cesarean section was warranted when intrapartum fetal heart rate decelerations were detected in one of the twins. Cord entanglement was noted at the time of delivery. Ultrasonography at week 29 had failed to demonstrate a partitioning membrane but as late as week 36 did not visualize cord entanglement. The diamniotic environment became monoamniotic from an unknown event some time in the late second or third trimester of pregnancy. This report emphasizes that ultrasonographic visualization of a partitioning membrane does not preclude future cord accidents.