Chorioamnionicity is a crucial factor in the risk assessment of a multiple pregnancy. Although knowledge of zygosity is of limited clinical importance, the early identification of the number of fetuses and determination of their chorionicity and amnionicity is critical to proper obstetric management. Prenatal ultrasound permits early and accurate diagnosis of chorioamnionicity. The sonographic features of monochorionicity versus dichochorionicity in the early first trimester, late first trimester, and second and third trimesters share many similarities but significant variation occurs with advancing gestation. While some sonographic findings have very high predictive values and can be confidently used to establish chorioamnionicity, others are less reliable. Therefore, evaluation of as many features as possible should be performed during obstetric ultrasound.