Monochorionic (MC) twin and triplet pregnancies pose complex clinical problems and high risks of types that are not seen in dichorionic (DC) twin pregnancies. Fetal growth discordance (FGD) and twin-to-twin transfusion syndrome (TTTS) are the most common problems. The MC twin placenta is usually a truly single, not fused, placenta that is produced by a single zygote and intended for the metabolic support of a singleton fetus. There are three vascular consequences of the insertion of two or more umbilical cords into an MC placenta: cord insertions, single umbilical artery, and interfetal vascular connections. TTTS is usually caused by a relatively low number of small diameter arteriovenous connections (AVCs) in combinations that result in net chronic blood seepage into the recipient twin. All types of vascular connections are involved in major complications in MC twins, including TTTS, donor/recipient role reversal after treatment, and neurological damage of a single surviving fetus.