Approximately 1% to 3% of all pregnancies in the United States are multiple gestations. The vast majority (97-98%) are twin pregnancies. Multiple pregnancies constitute significant risk to both mother and fetuses. Antepartum complications-including preterm labor, preterm premature rupture of the membranes, intrauterine growth restriction, intrauterine fetal demise, gestational diabetes, and preeclampsia-develop in over 80% of multiple pregnancies as compared with approximately 25% of singleton gestations. This article reviews in detail the maternal physiologic adaptations required to support a multiple pregnancy and the maternal complications that develop when these systems fail or are overwhelmed.