Unlike twin-to-twin transfusion syndrome, which arises when twin oligohydramnios polyhydramnios sequence (TOPS) is present, TAPS develops in its absence.

[4][5] The cause of TAPS is slow and persistent unbalanced feto-fetal transfusion through tiny placental anastomoses, which progressively results in highly discordant hemoglobin levels.

A slow transfusion of blood from the donor to the recipient is made possible by the few tiny anastomoses, which eventually cause very disparate hemoglobin levels.

The presence of polycythemia in the recipient and (chronic) anemia in the donor, along with characteristic placental angioarchitecture as determined by injection with colored dye, are the basis for the postnatal diagnosis of TAPS.

[6] Weekly ultrasound monitoring, which includes a full staging Doppler examination with the MCA-PSV, is part of the expectant management protocol.