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Twin Reversed Arterial Perfusion (TRAP) Sequence (Acardiac Twin)

Twin Reversed Arterial Perfusion (TRAP) Sequence (Acardiac Twin)

What is Twin Reversed Arterial Perfusion (TRAP) Sequence?  

Twin reversed arterial perfusion (TRAP) sequence is a rare, serious complication that occurs in pregnancies with identical twins who share a placenta, known as monochorionic twins.

In TRAP sequence, one twin develops normally while the other is severely malformed, generally missing an upper body, head, arms and a heart. This malformed fetus is referred to as an “acardiac” twin, which means “no heart”; although in rare cases a primitive heart may be present.  The healthy fetus is known as the “pump” twin because it pumps blood to the acardiac twin.

TRAP sequence gets its name from the unusual blood flow pattern that occurs. When the healthy twin pumps blood to the acardiac twin, it flows through the umbilical artery of the “acardiac” twin that usually carry blood in the opposite direction. This backward blood flow is known as “reverse arterial perfusion.” 

TRAP sequence is sometimes referred to as “acardiac twinning.” The condition can also occur in pregnancies involving monochorionic triplets or higher-order multiples.

How does TRAP sequence affect my babies?

The increased demands on the heart of the pump twin increases the risk of serious pregnancy complications and puts the healthy twin at risk of heart failure and death. The larger the acardiac twin grows, the greater the risks.

Complication rates in TRAP pregnancies are high and include:

  • Too much amniotic fluid, known as polyhydramnios as increased blood flow increases fetal urine production, the main component of amniotic fluid
  • Preterm labor and birth, when excess amniotic fluid causes the uterus to enlarge
  • Hydrops in the pump twin, a life-threatening condition in which fluid builds up in two or more areas of the baby’s body, causing severe swelling
  • Heart failure in the pump twin
  • Miscarriage
  • Stillbirth
  • Death of the pump twin shortly after birth, due to heart failure or other complications

In severe cases, fetal intervention may be recommended to increase the pump twin’s chance of survival.  

The severely malformed acardiac twin lacks the vital organs necessary for survival outside of the womb. While not fully understood, the myriad of abnormalities in the “acardiac” twin are believed to be due to reduced blood flow, which prevents proper development of several organs and anatomic structures in  the “acardiac” twin.


TRAP sequence is rare, historically estimated to occur in 1% of monochorionic twin pregnancies or 1 out of every 35,000 births.

However, due to earlier detection in the first trimester of pregnancy and an increased number of pregnancies with multiples due to assisted reproductive technologies, the condition is now estimated to occur in 2 to 3% of monochorionic twins.


TRAP sequence is typically diagnosed during a routine prenatal ultrasound. The ultrasound will reveal if the fetuses are sharing the same placenta (monochorionic) and if one twin is malformed and lacking cardiac activity. An excess amount of amniotic fluid may also be detected.

If TRAP sequence is diagnosed or suspected, you may be referred to a fetal center for a comprehensive evaluation and specialized care.

At Texas Children’s Fetal Center, we arrange for you to visit as quickly as possible to meet with a team of specialists experienced in the diagnosis and treatment of this rare condition, including maternal-fetal medicine (MFM) physicians, fetal imaging experts, fetal cardiologists, and neonatologists.

You will undergo additional testing to help us gain critical information, including:  

  • High-resolution anatomy ultrasound to confirm the diagnosis, evaluate the condition and look for other abnormalities
  • Ultra-fast MRI for a more detailed view of fetal anatomy
  • Fetal echocardiogram to assess fetal heart structure and function to evaluate the risk for heart failure in the pump twin
  • Doppler ultrasound to examine blood flow patterns in both twins
  • Amniocentesis and chromosomal analysis to screen for genetic abnormalities

Following this detailed assessment, our specialists will meet with you about the results, discuss treatment options, and counsel you about the potential risks and benefits, to help your family make the most informed decisions.

Treatment During Pregnancy

Your pregnancy will be closely monitored with frequent ultrasounds and fetal echocardiograms to assess the heart health of the pump twin, the size of the acardiac twin, and amniotic fluid levels.

The goal is to reduce the risk of heart failure and death in the pump twin. If the pump twin’s survival is at risk, and it is late enough in gestation, early delivery may be recommended.

If the pump twin is at imminent risk of heart failure but it is too early for delivery, fetal surgery may be recommended.

Fetal Surgery

Fetal surgery is performed to stop blood flow to the acardiac twin, increasing the pump twin’s chance of survival. These procedures generally yield an 80 percent success rate.

For patients who are candidates, the timing and type of fetal intervention is highly individualized based on the gestational age at TRAP diagnoses as well as based on other maternal and fetal factors. Fetal surgery options include:

  • Radio Frequency Ablation (RFA), which uses radio frequency waves or bipolar energy (heat) to disconnect the shared blood vessels, sealing off the blood flow to the acardiac twin. This ultrasound-guided procedure, performed under local anesthesia, uses a thin needle placed through the mother's abdomen and uterus to apply the heat energy inside the “acardiac” twin.
  • Selective fetoscopic laser photocoagulation, a minimally invasive surgical procedure that uses a small camera (fetoscope) to locate and cauterize or disconnect the shared blood vessels in the placenta using a laser.
  • Intra-fetal Laser therapy, which also uses laser energy but inside the “acardiac” twin. A needled is directed under ultrasound guidance inside the “acardiac” twin and a laser fiber is used to stop the blood flow.
  • Umbilical cord occlusion using bipolar forceps, which uses electrical energy to stop the flow in the umbilical artery and vein of the “acardiac” twin

Following fetal surgery, close observation and frequent monitoring will continue for the remainder of the pregnancy. In some cases limited physical activity or medications may be used to reduce the risk for premature delivery.


Delivery should take place at a hospital with the expertise and resources required to treat these high-risk pregnancies and newborns, including the highest level neonatal intensive care unit (NICU), if needed. After fetal surgery, the “acardiac” twin will be delivered at the time that the pump twin is delivered.

Delivery and postnatal care should be carefully planned and coordinated across a team of maternal-fetal medicine, neonatal and pediatric specialists experienced in pregnancies involving TRAP sequence.

Our Fetal Center team works closely with pediatric experts from Texas Children’s Hospital, consistently ranked one of the best children’s hospitals in the nation, providing our patients seamless access to the critical care services and specialists their child may need after birth, avoiding the need for transfers. Here, the pediatric specialists responsible for treating your child have been an integral part of their care team since before birth.

Postnatal Care Team

Treatment needs after birth vary based on the health of the pump twin, gestational age at delivery, and any fetal intervention performed.

In most cases, babies with no complications following fetal intervention are born healthy with no long-term issues.

Possible postnatal care team members include:

Why Texas Children’s Fetal Center?

  • A single location for expert maternal, fetal and pediatric care. At Texas Children’s Hospital, mothers and babies receive the specialized care required for the diagnosis and treatment of TRAP sequence all in one location, including immediate access to our level IV NICU, avoiding the need to transport your critically ill newborn.
  • A skilled, experienced team with proven outcomes. We have a dedicated team of maternal-fetal medicine specialists, fetal imaging experts, fetal cardiologists, fetal surgeons, and neonatologists who work in concert to care for your family every step of the way, using proven protocols we’ve developed over the years. With their combined expertise and unified approach, this team offers the best possible care for these rare, complex pregnancies.
  • We care for children’s needs at every stage of life. Our comprehensive approach starts with your first prenatal visit and continues through your baby’s delivery, postnatal care, and beyond, as needed, thanks to one of the nation’s leading teams of fetal and pediatric specialists.

For more information or to schedule an appointment,

call Texas Children’s Fetal Center at 832-822-2229 or 1-877-FetalRx (338-2579) toll-free.

Our phones are answered 24/7. Immediate appointments are often available.