Twin Reversed Arterial Perfusion (TRAP) Sequence

Twin reversed arterial perfusion (TRAP) sequence is a rare condition that affects pregnancies with identical twins who share a placenta. The syndrome occurs when one of the twins lacks a functioning heart and gets blood from the other twin.

What is Twin Reversed Arterial Perfusion (TRAP) Sequence?

Twin reversed arterial perfusion (TRAP) sequence is a rare condition that affects pregnancies with identical twins who share a placenta. This is known as monochorionic twin pregnancies. The syndrome occurs when one of the twins, called the acardiac twin, lacks a functioning heart and gets blood from the other twin, called the pump twin. The heart of pump twin works twice as hard, which can place it at risk for heart failure. 

Without treatment, the healthy twin has less than a 50 percent chance of living. The malformed twin, often missing an upper body, head and limbs, cannot survive outside the womb.

TRAP sequence occurs in about 1 percent of monochronic pregnancies and 1 in 35,000 births. 

What are the Symptoms and Complications Associated with TRAP?

When this defect occurs, blood enters the malformed twin through the umbilical artery. Blood leaves through the umbilical vein, which normally carries blood from the placenta to the fetus. Blood flows in the opposite direction, or in reverse, giving the condition its name.

TRAP sequence greatly increases the risk for pregnancy problems (complications) and puts the life of the pump twin in danger. The larger the acardiac twin grows, the greater the risk.

Complications may include:

  • Extra amniotic fluid
  • Preterm labor and delivery
  • A life-threatening buildup of fluid in the pump twin’s body, called hydrops
  • Heart failure in the pump twin
  • Miscarriage or stillbirth
  • Death of the pump twin shortly after birth

What Causes TRAP?

The cause of TRAP sequence is not clear. However, it may be linked to changing blood pressure and blood flow during early pregnancy, which prevents the abnormal twin from developing a heart. The condition seems to happen randomly.

How is TRAP Diagnosed?

TRAP sequence is typically diagnosed during a prenatal ultrasound. This shows:

  • that the twins share one placenta and confirms a monochorionic twin pregnancy
  • one twin is growing normally while the other does not have any heart activity
  • a lot of extra amniotic fluid.

If these conditions occur and TRAP sequence is suspected, you may be referred to a fetal center for special testing and care. Other diagnostic procedures may include:

  • Anatomy ultrasound – A high-resolution ultrasound to confirm the diagnosis, look at the condition and find other problems.
  • Doppler ultrasound – A test for looking at blood flow patterns in both twins.
  • Fetal MRI – Non-invasive imaging test to get a clear, more detailed image of the fetuses.
  • Fetal echocardiogram – A special ultrasound to look at the twins’ heart structure and function while assessing risk of heart failure in the pump twin.
  • Amniocentesis/chromosome studies – A medical procedure in which a small amount of amniotic fluid is taken and studied to screen for genetic problems.

Since advanced imaging techniques, such as 3-D and 4-D ultrasound, are now available, TRAP sequence can be diagnosed as early as 11 weeks of gestation.

How is TRAP Treated?

Your pregnancy will be closely monitored by a fetal medicine specialist. You will have frequent testing, including ultrasounds and fetal echocardiograms. These tests will help your doctor look at the heart function of the pump twin, the growth of the acardiac twin and the level of amniotic fluid. 

If your doctor thinks there is an increased chance of heart failure for the pump twin, early delivery or fetal surgery may be recommended before or after the third trimester. If it is too soon for delivery and the pump twin’s survival is at risk, your doctor may do fetal surgery to cut off the blood supply to the acardiac twin.

Fetal surgery techniques include:

  • Radiofrequency ablation – This is an ultrasound-guided procedure, performed under local anesthesia. It uses radio frequency heat energy to cut off the blood vessels feeding the acardiac twin. The energy goes through a thin needle that is inserted through the mother’s belly (abdomen) and uterus and into the umbilical cord of the acardiac twin. This is an option when the gestational age is more than than 16 weeks.
  • Selective bipolar cord coagulation – When the gestational age is under 16 weeks, selective bipolar coagulation may be done. In this procedure, a tiny camera and surgical instrument are inserted into the mother’s abdomen and uterus to disconnect the shared blood vessels. This blocks the flow of blood to the acardiac twin. This procedure is minimally invasive.
  • Intra-fetal laser therapy – This ultrasound-guided surgery uses laser energy stop the blood flow inside of the acardiac twin. .
  • Umbilical cord occlusion using bipolar forceps – This technique uses electrical energy to stop blood flow in the umbilical artery and vein of the acardiac twin.

These procedures are minimally invasive. While each of these procedures involves risks for both mother and baby, including preterm labor and delivery, overall success rates are high. Your doctor will talk to you about which procedure is best for you.

Delivery with TRAP

If there are no signs of heart failure or maternal health problems, TRAP sequence does not need an early or Cesarean (C) section delivery. However, the delivery should be done in a hospital that is prepared to treat newborns with rare birth defects and mothers with high-risk pregnancies. This makes sure that the baby and mother have access to a team of neonatal and pediatric specialists, as well as a neonatal intensive care unit (NICU).

In many cases, the twins will be delivered at the same time.

What are the Long-term Effects of TRAP?

When close prenatal monitoring and needed fetal interventions are done, the pump baby is born healthy in most cases. Treatment may be done if the baby is delivered early or has problems at birth. However, most pump twins do not have any long-term health problems associated with the birth defect.