Twin to twin transfusion syndrome (TTTS) is an abnormality of the placental vasculature during pregnancy which affects identical twins. Identical twins share a common monochorionic placenta. If the placenta contains abnormal blood vessels the umbilical cord blood flow and the circulations of the fetuses becomes connected. As a result one fetus, known as the donor, receives less blood flow and the nutrients it contains which the other fetus, known as the recipient, receives more blood flow and the nutrients it contains.
At birth, both infants may have problems depending on the severity of the transfusion syndrome. The donor twin may have too little blood and need a blood transfusion, while the recipient twin may have too much blood and may need to have the amount of blood in his or her body reduced. While the infants are identical twins, they may have significantly different birth weight as the donor is frequently small from gestational age due to the lack of nutrients during intrauterine life which the recipient is bigger as a result of the increased volume of blood in the circulatory system.