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What is twin reversed arterial perfusion (TRAP)?

Twin reversed arterial perfusion (TRAP) sequence is a rare complication of identical monochorionic twin pregnancies (a pregnancy where twins share the placenta). With TRAP, one twin (known as the acardiac twin) does not develop a functional heart, or their heart is severely underdeveloped. This twin receives its blood supply from the other twin, the “pump twin,” who has a properly functioning heart. Because the twins share a placenta and their circulatory systems are connected, the pump twin supplies blood to itself and the acardiac twin, putting extra strain on its heart.

Characteristics of TRAP:

  • The twins are joined by a large blood vessel between their umbilical cords.
  • The acardiac twin is not compatible with life.
  • The pump twin’s heart works extra hard to pump to the acardiac (without a heart) twin, increasing the pump twin’s risk of heart failure and death.
  • In addition, if both twins are in the same amniotic sac (usually), they may get entangled and strangle the umbilical cord, resulting in sudden death of the pump twin.

At Boston Children’s Hospital, our Maternal Fetal Care Center specializes in the evaluation and treatment of TRAP and other complex fetal conditions.

Why and how often does TRAP occur?

We don’t know why TRAP occurs, but the rare condition is due to an abnormal development of one of the fetuses, resulting in a severely malformed twin that lacks critical organs — including but not limited to a functional heart (acardiac) — and relies on its developing co-twin for blood supply. TRAP occurs in about 1 percent of identical twin pregnancies.

Twin Reversed Arterial Perfusion (TRAP) | Diagnosis & Treatments

How is TRAP diagnosed?

Your doctor can diagnose TRAP based on a prenatal ultrasound.

Signs of TRAP include:

  • The pump twin is larger than the acardiac twin.
  • Polyhydramnios (excessive accumulation of amniotic fluid) developed by the pump twin and possibly heart failure, which can be seen on ultrasound.

If your doctor sees these signs of TRAP, you may also receive a fetal echocardiography, which looks directly at the heart of the pump twin to assess how it’s functioning.

How is TRAP treated?

Treatment options for TRAP may include:

  • Monitoring: Close monitoring of the pregnancy through ultrasound and other diagnostic techniques to assess the health and development of both twins.
  • Intervention: In severe cases, Boston Children’s most common approach to treating TRAP is to separate the circulation between the twins.This involves either obliterating blood supply to the acardiac twin by radiofrequency ablation or coagulating and ligating its umbilical cord. The specific approach will be considered by our fetal surgeons according to the findings documented by prenatal ultrasound. The procedure is performed during the second trimester through a single puncture in the mother's abdomen.

    This will increase the chance of survival in the pump twin by decreasing the potential for heart failure. There is an increased chance of preterm labor after this procedure, so you will be monitored closely through the rest of your pregnancy.
  • Delivery: In severe cases of TRAP, or if the pump twin’s health is at extreme risk, we may recommend earlier delivery of the twins to save the life of the pump twin.

Your prenatal ultrasound findings will determine the specific approach doctors take for your pregnancy.

Twin Reversed Arterial Perfusion (TRAP) | Programs & Services