The incidence of twins has increased steadily in the past 20 years, primarily due to fertility treatments. In the United States, twins account for approximately 1.5% of all pregnancies. Between 1980 and 1999, the overall multiple birth ratio increased 59%, from 19.3 to 30.7 multiple births per 1000 live births (p < .001), with rates among whites increasing more rapidly than among African-Americans. Women of advanced maternal age, especially those aged 30 to 34, 35 to 39, and 40 to 44, experienced the greatest increases (62%, 81%, and 110%, respectively).1 However, although perinatal mortality in singleton pregnancies has fallen over the last decade due to advances in fetal medicine and diagnostic techniques and improvement in neonatal care, similar declines have not been seen in multiple pregnancies; in those pregnancies, perinatal loss still remains six times higher than in singleton pregnancies. Maternal complications of multiple pregnancy include hypertension, preeclampsia, anemia, antepartum hemorrhage, and postpartum hemorrhage. In the fetus, increased abortion rate, single fetal demise, preterm birth, low birth weight, congenital anomalies, and intrauterine growth retardation may occur. In addition, other unique complications may occur in monochorionic twin pregnancies, including twin transfusion, twin embolization, conjoined twinning, and twin reversed arterial perfusion syndrome.2 This article discusses the twin-specific complications only.