Acardiac anomaly, twin reversed arterial perfusion (TRAP) sequence or chorioangopagus parasiticus are synonymous terms referring to a rare complication unique to monochorionic multiple pregnancies in which there is apparent lack of a well-formed cardiac structure in one fetus (the acardiac twin), which is abnormally perfused by a structurally normal co-twin (the pump twin) through a single superficial artery-to-artery placental anastomosis. The condition therefore results in arterial blood flowing in a retrograde fashion from the pump twin towards the affected fetus, and this underlying pathophysiology indicates the preferred use of the clinical term TRAP-sequence. Pathophysiologically, the acardiac fetus acts as a parasite that can only survive in-utero as it is haemodynamically dependent upon the pump twin. The principal perinatal problems being associated with the TRAP sequence are pump-twin congestive heart failure, polyhydramnios and severe preterm delivery, although intrauterine death of the pump twin has been reported even in the absence of such features.