Number of yolk sacs does not predict amnionicity in early first-trimester monochorionic multiple gestations

Objective To determine the relationship between amnionicity and number of yolk sacs before 11 weeks of gestation. Methods Twenty-two cases of monochorionic multiple pregnancy were scanned before 11 weeks of gestation. There were 21 sets of twins and one of triplets. Amnionicity was determined by visualization of a dividing amniotic membrane between the gestational sacs. The number of yolk sacs was recorded and compared with the presence or absence of a dividing membrane for all fetuses. Results In 17/20 (85%) cases of monochorionic diamniotic twins, two yolk sacs were seen. In 3/20 (15%) cases of monochorionic diamniotic twins, a single yolk sac was seen. In the one case of monochorionic diamniotic triplets, two yolk sacs were visualized. In one case of monoamniotic twins, a single yolk sac was observed. Conclusions In monochorionic pregnancies, the presence of two yolk sacs predicts diamnionicity. However, the use of the number of yolk sacs as a predictor of amnionicity may not be accurate in a small proportion of Patients. The diagnosis of monoamnionicity can be made only following a careful search for a dividing amniotic membrane. Copyright (c) 2005 ISUOG. Published by John Wiley & Sons, Ltd.