Twin-to-twin transfusion syndrome occurs specifically in identical twins sharing the same placenta. It is estimated to occur in 15% of identical twin pregnancies. In twin-to-twin transfusion syndrome, the twins share not only the same placenta but some of the same circulation.
This allows the transfusion of blood from one twin (the donor) to the other (the recipient). The donor twin becomes small and anaemic,
and the recipient twin becomes large and overloaded with blood.

Because the recipient twin has more blood, he/she also urinates
more and has more amniotic fluid. The donor twin has less
amniotic fluid; sometimes there is so little fluid that the fetus appears
on ultrasound to be stuck in place on the wall of the uterus
(known as "stuck twin phenomenon").