There is a higher incidence of perinatal mortality and morbidity in twins than in singleton pregnancies. The main reason for this increase is the higher incidence of preterm delivery in twins. There are special conditions unique to twins that also contribute to this increased perinatal morbidity and mortality. These conditions include monoamniotic twins, congenital anomalies unique to twins, ie, conjoined twins and acardia, intrauterine fetal demise, and twin-to-twin transfusion syndrome. These conditions are the subject of this review. The prenatal determination of chorionicity is discussed first, because this assessment is key to the diagnosis and management of many of these conditions.