Twin Reversed Arterial Perfusion Sequence

Twin reversed arterial perfusion (TRAP) used to be known as acardiac twin pregnancy. It is a very rare type of twinning that is only seen in pregnancies with only one placenta, happening in about 1 in every 35,000 identical twin births. 

C.S. Mott Children’s Hospital is home to one of the country’s most experienced Fetal Diagnosis and Treatment Centers. Our comprehensive experience with TRAP includes fetal diagnosis and counseling, individualized delivery planning and fetal intervention. 

Twin reversed arterial perfusion sequence overview

TRAP is a rare diagnosis in which one twin develops, and although there is a higher risk of birth defects in this twin, it is usually normal.  This baby is called the “pump twin.”  The other twin never develops as expected and gets all of its blood supply through the placenta from the other baby.   This is called the “TRAP fetus,” or sometimes referred to as the “acardiac fetus” because it either has a very underdeveloped heart or no heart at all.  It could not grow or survive without the blood supply from the normal twin and it will not survive after birth.  The term “reversed perfusion” is used because the blood enters the undeveloped twin through the vessels in the opposite direction.  Since the more normal twin is doing all of the work to pump blood for both babies, it is at risk for heart failure because it is working twice as hard to support both.  If the TRAP or acardiac fetus is large enough that there is concern it could cause heart failure in the pump twin, treatments are offered. The cause of TRAP is unknown, and it is not something we know how to prevent.

Diagnosis of twin reversed arterial perfusion sequence

The diagnosis of TRAP is made by ultrasound.  The size of both twins is an important factor used to determine the outcome of the normal twin.  If the abnormal twin’s weight is greater than 50% of the normal twin, the survival of the normal twin is only 10%.  A fetal echocardiogram may be used to evaluate the heart function of the normal twin to see if heart strain is present.

An amniocentesis is offered to analyze the chromosomes of each twin. About 9% of the normal twins, and 30% of the acardiac twins will have a chromosomal abnormality.

Management of pregnancy

Regular ultrasounds are used to monitor how well the normal twin is doing. Ultrasounds will also be used to check the growth and blood flow to the acardiac twin.  Some acardiac twins are small and the blood supply to it stops on its own. When this happens, the normal twin can grow without the side effects of being a pump twin. An acardiac twin measuring greater than 50% the size of the normal twin threatens the life of this normal baby.  Fetal intervention is offered when the acardiac twin meets this criteria.

Fetal treatment of twin reversed arterial perfusion sequence

Treatment is available when the acardiac twin is very large in size, or the normal twin is showing signs of heart problems. There are two types of interventions available: Radio frequency ablation (RFA) and fetoscopic cord occlusion of the acardiac twin. Each pregnancy is unique. Your doctor will discuss the benefits of each intervention with you.

Delivery of a baby with twin arterial reversed perfusion sequence

Delivery should be at a hospital that is prepared for high-risk deliveries and has a neonatal ICU.  Although babies with TRAP are at risk for preterm delivery, they do not necessarily need a cesarean delivery.   Their health is usually dependent on the gestational age at birth and any cardiac issues from the acardiac twin.  How the normal twin does in the long term depends on their gestational age at birth.

University of Michigan is one of only a few centers nationwide at which the birth center is co-located within a comprehensive children’s hospital. This unique setting allows for seamless integration between our private-room birth center, state-of-the-art newborn intensive care unit (NICU), and access to around-the-clock pediatric surgical services in one convenient location.

Future Pregnancy Risk

TRAP is a rare condition with no familial link identified. An amniocentesis will provide karyotype information on both twins that could be a factor in future pregnancies. Your doctor and a genetic counselor will review the risk in your family.

Make an Appointment

Schedule an appointment by calling us at 734-763-4264.