Definition of Twin-to-twin transfusion syndrome:

Twin-to-twin transfusion syndrome (TTTS) is a rare condition that occurs only in identical twins while they are in the womb.

Causes, incidence, and risk factors:

TTTS occurs when blood moves from one twin to the other. The twin that loses the blood is called the donor twin. The twin that receives the blood is called the recipient twin.

Both infants may have problems depending on the severity of the transfusion. The donor twin may have too little blood, and the other may have too much blood. The donor twin may need a blood transfusion, while the recipient twin may need to have the amount of blood in his or her body reduced.