A total of 66 twin pregnancies was studied antenatally with ultrasound, followed up clinically, and the placentae examined pathologically to determine if, solely on the basis of antenatal sonography, an accurate assessment of amnionicity and chorionicity could be made. Sonographic features noted included number of placental sites, whether a membrane separating the fetuses could be visualized, fetal position and amount of amniotic fluid, and in some cases fetal positioning vis-a-vis each other and the uterine wall. Documentation of two placental sites confirms the presence of a dichorionic and, therefore, a diamniotic pregnancy. Documentation of a membrane separating the fetuses confirms the presence of a diamniotic pregnancy; lack of visualization of a separating membrane does not reliably predict, but does suggest, a monoamniotic pregnancy. Other features frequently assist in prediction of amnionicity, chorionicity, and zygosity; the most helpful of these was the "stuck twin" sign, in which one twin moved freely but the other remained in a fixed position adjacent to the sidewall of the uterus despite changes in maternal positioning, indicating the presence of a nonvisualized membrane. Presence of absence of entanglement or intermingling of fetal parts or umbilical cords may also be helpful. In the absence of such sonographic features, antenatal sonographic diagnosis of a monoamniotic pregnancy cannot be made reliably.