Ultrasonographic criteria for the prenatal diagnosis of placental chorionicity in twin gestations.

Thirty-two patients with uncomplicated twin pregnancies had ultrasonographic examinations for genetic amniocentesis, confirmation of twinning, or assessment of fetal growth. The dividing membranes between the fetuses were visualized, and the thickness of the membranes was measured. With a thickness of 2 mm used as a cutoff point, the accuracy in predicting monochorionic or dichorionic twinning was 82% and 95%, respectively. Prenatal assessment of these dividing membranes may be helpful in the management of twin gestations.