In TRAP sequence, one twin is usually developmentally normal and the other twin has a serious condition, either missing a heart (acardiac) or a head (acephalic) or both, that prevents it from surviving on its own. As a result, the acardiac/acephalic twin receives all of its blood from the normal or "pump" twin.

Twin Reversed Arterial Perfusion (TRAP) sequence is a rare complication specific to monochorionic twins (twins sharing a single placenta). A severely deformed “acardiac” twin who often lacks a heart is supported by a co-twin who pumps blood to it across a shared placental artery.

Twin reversed arterial perfusion (TRAP sequence) is a rare condition of monochorionic twin pregnancies. It arises when the cardiac system of one twin does the work of supplying blood for both twins. The twin supplying the blood is known as the "pump twin" and develops normally in the womb.

In TRAP sequence, one twin is developmentally normal (pump twin) while the other develops without a functioning heart (acardiac twin) and many other structures (head, limbs, etc.) that would allow this twin to develop into a normal fetus and newborn.

TRAP sequence is typically treated using a minimally invasive fetal surgery. The goal of fetal surgery is to interrupt the blood supply from the pump twin to the nonviable twin. This reduces the strain on the pump twin’s heart and increases his or her chance of survival.