Some of the blood vessels of the other baby look like a fork in the road and split. They then cross their shared placenta over to the recipient twin. Because the recipient baby is getting his or her own blood, plus this extra blood, he or she becomes much larger in size. This extra blood is very thick like syrup Because the blood is so thick, the baby's heart must work harder to pump the blood which tires out the baby. The recipient twin urinates more because of the extra protein supply included with the extra blood.
Amniotic fluid is largely the babies urine.
Because of the extra urine, the amniotic fluid levels rise dramatically. This early warning sign can be deteced via ultrasound. There is very little amniotic fluid in the donor baby's sac. Because of the lower volume of blood to this baby, he or she can become very anemic. If the donor baby survives birth they typically do better than the recipient twin since they have not stessed their heart and are not as tired at delivery.