ABSTRACT
A total of 31 twin pregnancies were studied antenatally with ultrasound, followed up clinically, and the placentas examined to determine if, solely on the basis of prenatal sonography, an accurate assessment of amnionicity and chorionicity could be made. Sonographic features noted included number of placental sites, fetal gender, qualitative and quantitative evaluation of the dividing membrane. Documentation of two placental sites or different fetal gender confirms the presence of a dichorionic-diamniotic pregnancy. When only one placental site or similar fetal gender is demonstrated, examination of the membrane is helpful for distinguishing between dichorionic and monochorionic diamniotic gestations. A thick membrane separating the fetuses was seen in all dichorionic diamniotic pregnancies. A thin membrane was visible in 94.4 per cent of monochorionic-diamniotic pregnancies. The predictive value of a thick membrane with regard to a dichorionic-diamniotic pregnancy was 92.3 per cent and the predictive value of a thin membrane with regard to a monochorionic-diamniotic pregnancy was 100 per cent. The thickness of the membrane was measured in 13 cases. With a thickness of 2 mm used as a cutoff point, the accuracy in predicting monochorionic or dichorionic twinning was 100 per cent for both. Lack of visualization of a separating membrane suggests a monochorionic-monoamniotic pregnancy.