The ultrasonographic prediction of chorionicity in twin gestation.

In a prospective study of 69 consecutive twin gestations, chorionicity was assessed ultrasonographically, by the new technique of counting the number of layers visualized in the dividing membrane. For purposes of the study the antenatal categorization of chorionic type was based only on the number of layers observed. The pregnancy was classified as monochorionic when only two layers were identified and as dichorionic if three or four layers were seen. Confirmation of chorionic type was obtained after delivery by histopathologic examination of the placenta. The ultrasonographic technique used correctly determined chorionicity in 68 of 69 twin pregnancies. The predictive accuracy was 100% for 51 pregnancies designated by ultrasonography as dichorionic and 94.4% for 18 pregnancies considered to be monochorionic. These results suggest that counting the number of layers seen in fetal membranes by ultrasonography examination is an accurate way of determining chorionicity in twin gestation.