Monochorionic (MC) twin pregnancies may develop significant complications, and twin-to-twin transfusion syndrome (TTTS) has become among the best known to obstetricians and patients alike. A significant percentage of patients referred for suspected TTTS have a different underlying pathologic condition, however, and differentiating the subcategories of MC pathophysiologic conditions may change treatment course and outcome. The key to understanding complicated MC pregnancies lies in the placental angioarchitecture and intertwin vascular communications between the fetuses.