ABSTRACT

Abnormal placental vessels are common in monochorionic (MC) twinning. In the usual setting, blood from the placenta enters the fetal circulation through the umbilical veins and exits via the umbilical artery. Very rarely (1% of MC twins or 1:35 000 births), retrograde or reversed arterial perfusion takes place, from the placenta through the umbilical artery of one of the twins. Because the circulations of the twins are connected by arterioarterial and venovenous placental anastomoses, the twin with the reversed flow receives all of its blood supply from a normal co-twin who gains circulatory predominance-the so-called ‘pump’ twin. This vascular abnormality is currently termed the twin reversed arterial perfusion (TRAP) sequence.