#TRAP sequence

• Acardiac — Lacking a heart, or possessing a rudimentary, completely non-functional heart. In neonatology and fetal medicine, the term describes an 'acardiac fetus' (or acardiac twin), an extremely rare congenital anomaly occurring exclusively in monochorionic twin pregnancies due to Twin Reversed Arterial Perfusion (TRAP sequence). In this condition, the malformed fetus survives in utero only because it receives blood pumped by the heart of its healthy co-twin (the pump twin).
• Acardia — A congenital absence of the heart or a complete cessation in the ontogenetic development of the cardiac structure in a fetus. This severe structural anomaly is incompatible with independent extrauterine life and is documented almost exclusively within monochorionic multiple pregnancies with asymmetric development (TRAP sequence), where the affected fetus's blood circulation is maintained solely by its twin's heart through placental vascular anastomoses.