The prenatal identification of various types of placentation is important for the antenatal management of twin pregnancies. Such classification can be achieved with the use of ultrasound examination. In 32 cases of twin pregnancy the intervening septum was classified as either a thick (dichorionic) or thin (monochorionic) type. Macroscopic and microscopic examination of the placenta after delivery confirmed the ultrasound diagnosis in 24 of the 30 cases, an accuracy of 80%.