Membrane thickness in ultrasound prediction of chorionicity of twin gestations.

A retrospective review of sonograms performed on 75 twin gestations was performed to evaluate the ability of sonography to distinguish monochorionic from dichorionic gestations based on the thickness of the membrane separating the fetuses. Clinical or pathologic evidence of chorionicity and amnionicity was available in all cases. A thick membrane had a predictive value of 83% for dichorionicity and was seen in 89% of the first sonograms obtained on dichorionic gestations. Of third trimester dichorionic pregnancies, a thick membrane was seen in only 52%. A thin membrane on the initial study had a predictive value for monochorionic diamniotic pregnancy of 83%, but was seen in only 54% of cases. There was 100% intraobserver and 91% interobserver concordance in interpretation of membrane thickness. Technical factors important in interpretation of membrane thickness are discussed. The appearance of the membrane can be useful in sonographic evaluation of chorionicity and amnionicity in twin gestations, but should be used in conjunction with all other information available.