Twin-To-Twin Transfusion Syndrome

Twin-to-twin transfusion syndrome (TTTS) develops only when twins share a placenta. The clinically important form is usually in a monochorionic diamniotic pregnancy, in which each fetus has a separate amniotic sac but both circulations communicate through placental vascular anastomoses. Some anastomoses carry blood predominantly from one fetus to the other. When bidirectional connections do not adequately balance these channels, the net flow becomes donor-to-recipient. The donor gradually loses circulating volume. The recipient receives more blood than the cardiovascular system can comfortably handle. The donor’s reduced renal perfusion causes less urine production, so the donor sac becomes oligohydramniotic. The fetus may appear compressed against the uterine wall, and the bladder becomes small or intermittently difficult to see. Persistent hypovolaemia can progress to growth restriction, abnormal Dopplers, hydrops, and death. The recipient’s increased circulating volume stimulates increased urine production, creating polyhydramnios. The excess volume also increases cardiac workload. Cardiomegaly, ventricular hypertrophy, atrioventricular valve regurgitation, abnormal ductus venosus or umbilical venous flow, and high-output cardiac failure may follow. These cardiac findings can be clinically significant even when the Quintero stage has not yet...