Children's Hospital Colorado

Twin Reversed Arterial Perfusion Sequence (TRAP Sequence)

What is TRAP sequence?

Twin reversed arterial perfusion sequence, or TRAP sequence, occurs in twin pregnancies in which one of the babies does not fully form and has either a malformed or absent heart (acardiac twin). This then puts stress on the heart of the sibling twin, called the "pump" twin, because his or her heart is working for two.

In an acardiac twin pregnancy, the normal twin's heart must work harder in order to pump blood to not only the placenta, but also to the acardiac twin. This blood flow is the reverse of what occurs in a normal twin pregnancy, where oxygenated blood is provided by the placenta, with the baby returning unoxygenated blood to the placenta. In this condition, the acardiac twin adds an additional burden on the normal twin's heart which may lead to heart failure.

What are the different kinds of TRAP sequence?

The type of TRAP sequence babies have is determined by how developed the abnormal twin is. For example, he or she may have a very abnormal heart or no heart at all. Some have a partial formation of the upper body and head and others may have none. Additionally, each TRAP sequence needs to be evaluated to determine what risk it places on the normal twin.

What causes TRAP sequence?

TRAP sequence occurs in 1 in 35,000 pregnancies and in less than 1% of identical twin pregnancies. TRAP sequence is caused by the development of one abnormal twin in an identical twin set, or in what is called a monochorionic twin pregnancy. The normal twin then supplies the blood flow for the abnormal twin, which may lead to heart failure and death.

Complications of TRAP sequence

Because so much work is required for the healthy twin to pump blood to the acardiac twin, it leaves the healthy twin at risk for heart failure and death.

Other complications can include polyhydramnios (excess amniotic fluid), which can cause swelling for the mother, as well as preterm labor and delivery. This has been documented in 46% of all TRAP sequence twins.

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