Monochorionic twins

In normal pregnancies each cotyledon is supplied by an umbilical artery that carries deoxygenated blood from the fetus to the placenta and a vein that carries oxygenated blood from the placenta to the fetus

In monochorionic twins most of the cotyledonos are normal (they are supplied by an artery and vein from the same fetus) but there are also abnormal vascular communications between the circulations of the two fetuses:

• Some of the cotyledons are shared by the two fetuses with an arterial supply from one of the twins and a venous drainage to the other cotwin
• There are also direct communications artery-to-artery and vein-to-vein between the two fetuses
• Imbalance net flow of blood across placental vascular communications from one fetus, donor, to other, the recipient, results in twin-to-twin transfusion syndrome (TTTS)