TRAP sequence: Twin-reversed arterial perfusion sequence. TRAP sequence, sometimes referred to as TRAP syndrome, is a condition that occurs only in identical twin pregnancies that share a placenta (known as monochorionic twins). In this condition, one twin is typically healthy, while the other has a serious condition that prevents it from surviving on its own, such as a missing heart (acardiac fetus) or head (anencephaly). This abnormal twin is kept alive because blood is pumped from the normal twin through the placenta, via arteries that pump blood in a 'reversed' direction to the abnormal twin. Because of the extra workload on the healthy twin's heart, the healthy twin is at risk for developing heart failure. TRAP sequence occurs in approximately 1% of monochorionic twin pregnancies and 1 in every 35,000 live births. Also sometimes referred to as acardiac twinning.