In the twin-to-twin transfusion syndrome, the donor twin is always anemic, owing not to a hemolytic process but to the direct transfer of blood to the recipient twin, who becomes polycythemic. The recipient may suffer thromboses secondary to hyper-transfusion and subsequent hemoconcentration. Although the donor placenta is usually pale and somewhat atrophied, that of the recipient is congested and enlarged. Hydramnios can develop in either twin but is more frequent in the recipient because of circulatory overload. When hydramnios occurs in the donor, it is due to congestive heart failure caused by severe anemia.