Monoamniotic (MA) twins occur in about 2–5% of monochorionic (MC) pregnancies. The condition is characterized by late splitting of the developing embryo at around 8–9 days after fertilization and may also occur in higher-order multiples. During the first trimester, ultrasound diagnosis in MC-MA twins is based on visualization of only a single aniotic cavity without an intertwin membrane, and can be confirmed by the presence of only one yolk sac. The visualization of cord entanglement confirms the diagnosis at any stage of pregnancy, and has important implications for further management. Criteria for the proper diagnosis of MC-MA multiple gestation and cord entanglement are given.