Twin-reversed arterial perfusion / TRAP sequence occurs only in the setting of a monochorionic gestation and complicates approximately 1% of monochorionic twin gestations, with an incidence of 1 in 35,000 births.

In the TRAP sequence, one twin is not viable, lacking a heart (acardiac) and defined anatomical features such as a head (acephalic). While in utero, the acardiac/acephalic twin receives all of its blood supply from the normal or "pump" twin.