Twin Reversed Arterial Perfusion Sequence (TRAP Sequence)

What is twin reversed arterial perfusion sequence (TRAP sequence)?

Twin reversed arterial perfusion sequence (TRAP sequence, also sometimes called TRAP syndrome) is a condition that occurs only in identical twins that share a placenta (monochorionic), in which one twin, lacking a functioning cardiac system, receives blood from the normally developing twin called the “pump twin.” This places an enormous demand on the heart, putting the pump twin at risk for cardiac failure. Left untreated, the pump twin will die in 50 percent to 75 percent of cases.

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.

“Reversed arterial perfusion” is used to describe this condition because blood flow is opposite from the normal blood supply of the fetus. In TRAP sequence, blood enters the abnormal twin (also called the acardiac or acephalic twin) through the umbilical artery (which usually carries blood away from the fetus back to the placenta) and exits through the umbilical vein, which normally carries blood from the placenta to the fetus.

TRAP sequence affects approximately 1 percent of monochorionic pregnancies, with an incidence of 1 in 35,000 births.

Causes of TRAP sequence

TRAP sequence is a rare complication of identical twinning. Although the initiating events are unclear, TRAP sequence is thought to result from communication between the blood vessels in the two fetal circulations as well as a cardiac problem in the recipient twin. The condition does not appear to run in families and there are currently no reports of recurrence in the same family.

Diagnosis of TRAP sequence

Because of the strain on the pump twin's heart, every case of TRAP sequence should be evaluated by fetal echocardiography. The reversed arterial flow in the acardiac/acephalic twin should be documented using color flow Doppler.

It is important to exclude a chromosomal abnormality prior to offering a fetoscopic procedure in TRAP sequence since the incidence of chromosomal abnormality in the pump twin may be as high as 9 percent. Over half of these pregnancies are complicated by polyhydramnios and 75 percent by preterm labor.

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.

Treatment for TRAP sequence

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. At The Children's Hospital of Philadelphia, we offer selective bipolar cord coagulation and radiofrequency ablation (RFA) procedures as treatment options for mothers carrying twins diagnosed with TRAP sequence.

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.

Selective bipolar cord coagulation procedure

The entire selective bipolar cord coagulation procedure is performed using small, hollow needles called trochars. These are inserted through the mother’s abdomen. In most cases, your medical team will instruct you to arrive at the hospital on the morning of the procedure, which is performed in the operating room. Mothers receive conscious sedation (IV sedation) and numbing medication at the site on the abdomen.

Your abdomen is prepped with an antibacterial solution and a trochar is placed into the amniotic space to allow the introduction of instruments, such as a small camera (fetoscope) and a coagulation device into the amniotic cavity. The coagulation device will be used to block blood flow to the non-viable twin.

After the surgery, you will be transferred to the Garbose Family Special Delivery Unit for a typical post-operative stay of approximately six hours. 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. You will be discharged the same day of your procedure.

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

Volumes & outcomes

Our team at the Center for Fetal Diagnosis and Treatment cares for a high volume of monochorionic twin pregnancies each year. Since 1995, more than 3,817 complicated multiple gestation pregnancies have been referred to the Center for Fetal Diagnosis and Treatment, including 226 referrals for TRAP sequence.

This experience helps to continuously expand our understanding of these complex pregnancies and our ability to differentiate between conditions such as TRAP sequence, selective intrauterine growth restriction (SIUGR) and twin-twin transfusion syndrome (TTTS).

Twin Reversed Arterial Perfusion (TRAP) Sequence Experience

Since 1995, 182 pregnancies have been referred to the Center for Fetal Diagnosis and Treatment with suspected twin reversed arterial perfusion (TRAP) sequence. 80 pregnancies were treated for TRAP sequence using the procedures shown here:

Radiofrequency Ablation (RFA): 43
Bipolar Cord Coagulation (BCC): 37

Average gestation age for delivery, regardless of type of procedure, is 35 weeks.


Based on data from the Center for Fetal Diagnosis and Treatment at The Children's Hospital of Philadelphia, 1995 through June 2016.


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