Twin gestations make up approximately 1-2% of all pregnancies and are associated with a significant increase in morbidity and mortality for both the mother and the fetuses. Addtional characteristics of a twin gestation that alter the expected outcome for the fetusesare the chorionicity (number of placentas)and amnionicity (membranes). The perinatal mortality rates are approximately 10% in dichorionic, 25% in monochorionic,diamniotic and 50% in monochorionic,monoamniotic gestations. Identification is therefore important for planning prenatal surveillance as well as for counselling of the parents. Early, chorionicity can be determined by counting gestational sacs and amnionicity by counting the number of embryos within a single sac by assessing heart activity. Around 10 weeks, the placentas and membranes fuse making this assessment more difficult. As dichorionic pregnancies have a membrane composed of 2 layers of amnion and two layers of chorion, the intervening membrane is thicker than seen with a monochorionic, diamnionic pregnancy but this becomes less appearant later in pregnancy. If the placentas are not identifiable as seperate, a finding which indicates a dichorionic (or multichorionicity in multiple gestation) pregnancy is the extension of a triangular projection of the placenta between the membranes. This finding, referred to as the "twin peak sign" is not seen in monochorionic pregnancies.