“Twin reversed arterial perfusion (TRAP) sequence or acardiac twinning is a very rare problem, occurring in approximately 1% of monochorionic twins (MC, twins sharing one placenta). One twin is usually structurally completely normal. The other is an abnormal mass of tissue, consisting usually of legs and a lower body, but no upper body, head or heart. Because of the absent heart, the term “acardiac twin” has been used to describe this mass. The normal fetus is referred to as the “pump twin” because its heart is used to pump blood to the abnormal mass. The “acardiac twin” has no chance of survival.
Due to the absence of a beating heart, the acardiac does not send blood to any portion of the placenta, and all of its blood supply comes from and goes back to the circulation of the pump twin through unique vascular connections on the surface of the shared placenta. Arteries usually carry blood away from the fetus and toward the placenta to receive oxygen from the mother’s circulation. When there is an “acardiac twin”, the unique vascular connections allow blood in the artery to flow in the reversed direction (toward the acardiac fetus rather than away from it). Thus, the phrase “twin reversed arterial perfusion” (TRAP) sequence has been used to describe this condition.
The normal “pump” twin faces the excess burden of having to send and receive blood to the acardiac mass as well as to its own growing tissues. As such, the normal twin’s heart has to work extra hard and is under a lot of stress. This can result in heart failure for the normal twin. Left untreated, up to 50% of these otherwise normal twins may die in utero (stillbirth) or die shortly after birth.” Read More…