Twin Reversed Arterial Perfusion

Twin reversed arterial perfusion sequence (TRAP sequence) is a rare, but serious, complication of identical twinning. TRAP sequence is seen in approximately one percent of identical twins who share a placenta (monochorionic twins).

What causes this condition

TRAP sequence occurs when one twin (acardiac twin), lacking a cardiac system and severely deformed, receives blood from the normally developing twin (pump twin) through what is believed to be backward flow within the surface vessels of the placenta. This places an enormous demand on the heart of the pump twin, raising the risk of heart failure, as well as producing too much urine and amniotic fluid.

The risk to the pump twin rises proportionately to the increasing size of the abnormally developing twin. TRAP sequence is also associated with excess amniotic fluid, caused by increased blood flow to the kidneys that results in overproduction of fetal urine (the primary source of amniotic fluid). This buildup of fluid may cause polyhydramnios, leading to premature rupture of the fetal membranes and preterm labor.

The condition does not appear to run in families and there are currently no reports of recurrence in the same family.


Initial diagnosis for TRAP sequence is often by routine prenatal ultrasound.

The difference in fetal weight between the twins is predictive of outcome. When the acardiac/acephalic twin outweighs the pump twin by more than 50 percent, death occurs in 64 percent of cases. If the acardiac/acephalic twin outweighs the pump twin by more than 75 percent, death occurs in 95 percent of cases. Risk of pregnancy complication is related to the size of the abnormal fetus.

How this condition is treated

Left untreated, the pump twin will die in 50 to 75 percent of TRAP sequence cases. TRAP sequence requires occlusion and division of the cord due to the risk of cord entanglement. Fetal echocardiography is essential for monitoring the pump twin for signs of decompensation and possible intervention.

The goal of fetal surgery is to interrupt blood supply to the non-viable twin. This reduces cardiac strain on the pump twin and increases his or her chance of survival.

After Surgery

After the surgery, you will be transferred to the High Risk Unit. During this time, you and your baby are carefully monitored for complications. Mothers are typically on bed rest and receive medications to help prevent preterm labor.

After discharge from the hospital, you may return to your referring doctor for ongoing care and delivery.