Twin-twin transfusion syndrome (TTTS) is a rare disorder that sometimes occurs when women are pregnant with identical (monozygotic) twins. It is a rare disease of the placenta, the organ that joins the mother to her offspring and provides nourishment to the developing fetuses. During the development of identical twins, there are always blood vessels in the fetuses' shared placenta that connect their blood circulations (placental anastomoses). In most cases, the blood flows properly through these vessels. However, in twin-twin transfusion syndrome, the blood begins to flow unevenly, with one fetal twin receiving too much blood (recipient) and one receiving too little (donor). The recipient twin may experience heart failure due to continual strain on its heart and blood vessels (cardiovascular system). The donor twin, on the other hand, may experience life-threatening anemia, insufficient nutrition and oxygen due to its inadequate supply of blood. Such an imbalance in blood flow (i.e., twin-twin transfusion) can occur at any time during the pregnancy, including during delivery.
The effects of twin-twin transfusion syndrome can vary in severity from case to case, depending upon when during pregnancy the syndrome occurs, when it is diagnosed, and any treatment that may be given. The cause of this syndrome is not fully understood, although it is known that placental characteristics play an important role.