Prediction of Amnionicity Using the Number of Yolk Sacs in Monochorionic Multifetal Pregnancy

The purpose of this article was to evaluate the accuracy of predicting amnionicity using the number of yolk sacs by diagnostic ultrasound examination in monochorionic (MC) multifetal pregnancies between 7 + 0 and 9 + 6 gestational weeks. A total of 97 patients with MC multifetal pregnancies underwent early ultrasound examination from 2004 to 2014 at Cheil General Hospital and Women's Healthcare Center. All patients for whom the number of yolk sacs was reported were included in this study. We compared the number of yolk sacs with amnionicity confirmed by an intertwine membrane. Overall, there was a 9.3% (9 cases) discrepancy in number of yolk sacs and amnionicity (4.3% for monochorionic diamniotic, 36.4% for monochorionic monoamniotic, and 33% for monochorionic triamniotic). Among the 9 cases with discrepancies, 4 cases with 2 yolk sacs were confirmed as monoamniotic pregnancies and 4 MC twin pregnancies showing a single yolk sac were diagnosed as diamniotic twin pregnancies. One case with 2 yolk sacs was identified as a triamniotic triplet pregnancy. In 9.3% of MC gestations, the number of yolk sacs was not correlated with the number of amnions in our study. To determine amnionicity in MC multifetal pregnancies, we recommend careful evaluation not of the number of yolk sacs but the presence or absence of intertwine dividing membrane after 8 gestational weeks.