Because the recipient twin is getting his or her own blood, plus the extra blood, he becomes larger in size. This is one of the first signs of the syndrome. This extra blood is very thick like syrup. Because the blood is so thick, the baby's heart has to work harder to pump the blood. Because of the extra blood, the baby is urinating a lot.
Amniotic fluid is generated by the baby urinating.
This causes the recipient twin to have too much amniotic fluid. This baby is getting too little blood and thus, does not urinate very much. This is why there is very little fluid in his or her amniotic sac. When born, it is often the donor twin who does better, because they are not as tired. Their heart has not had to over exert itself pumping the extra blood.