In twin-to-twin transfusion syndrome, which statement accurately describes the donor and recipient twins?

• A. Donor twin becomes underperfused; recipient twin becomes overperfused with polycythemia
• B. Donor twin becomes overperfused; recipient becomes underperfused
• C. Both twins equally perfused
• D. TTTS only occurs in dichorionic twins

Answer: (A)

Twin-to-twin transfusion syndrome happens when twins share placental blood vessels in a monochorionic pregnancy, creating unbalanced blood flow between the fetuses. The donor twin loses blood to the recipient through placental anastomoses, becoming underperfused. This reduced blood flow leads to decreased urine production and oligohydramnios in the donor twin, with possible growth restriction. The recipient twin, on the other hand, receives excess blood, becoming overperfused and often displaying polyhydramnios due to increased urine output from the higher circulatory load. Hematologically, the recipient can become polycythemic because of increased red blood cell mass. This pattern—donor underperfused and recipient overperfused with possible polycythemia in the recipient—fits TTTS.

TTTS occurs with monochorionic placentation, not with dichorionic twins, which is why the statement about TTTS being limited to dichorionic pregnancies is not correct.