Acardiac Twin and Twin Reversed Arterial Perfusion (TRAP)

What is TRAP?

Twin reversed arterial perfusion syndrome (TRAP), also called “acardiac twin”, can happen if one of an identical twin pair does not develop normally very early in the pregnancy. It is a rare complication of identical twins. (Identical twins share a single placenta, or afterbirth. They start as a single embryo, that splits to form twins.)

The underlying cause of TRAP is unknown. It is identified by ultrasound: a twin pregnancy is seen, but one of the twins has no beating heart – yet blood is flowing through its umbilical cord. Sometimes the abnormal twin will have recognizable upper body structures, such as arms. Most often, however, all that is seen is a swollen mass of tissue, with some leg-like structures.

Some believe that an acardiac twin is a result of abnormal blood flow between the twins: one twin pumps oxygen-poor blood through placental connections into the other twin. That fetus gets damaged by the lack of oxygen and does not develop a heart and upper body (often including the head, brain and arms). This abnormally formed twin remnant (or acardiac twin) can never survive. The healthy twin will pump blood through its own body, the placenta and the twin remnant, providing just enough oxygen and nutrition the to acardiac’s tissue mass for it to grow.

The term “twin reversed arterial perfusion” refers to direction of blood flow in the umbilical cord of the abnormal twin remnant: normally, blood flows from the beating heart through the umbilical arteries to the placenta, and returns from the placenta to the fetus through the umbilical vein. Because the umbilical arteries of both twins are connected, blood will be pumped by the healthy twin’s heart through its umbilical arteries, and backwards through the umbilical artery of the acardiac twin remnant.

What happens before birth?

TRAP is often diagnosed before birth. Early in pregnancy, it can easily been mistaken for a twin pregnancy with one healthy alive twin and a miscarriage (“demise”) of the other twin because there is no visible heart beat. The difference between the two is that, in TRAP, there is blood flow in the umbilical cord of the “demised” twin, and it flows in the “wrong” direction.

If the abnormal twin remnant is small, there are usually no problems during the pregnancy. If the twin remnant is fairly large, a lot of blood from the healthy twin will be pumped into it, causing heart strain in the healthy twin (this twin is often called the “pump twin”). This can lead to heart failure of the pump twin. Signs of heart failure in the fetus are increased amounts of amniotic fluid (polyhydramnios), swelling of the skin (edema), late miscarriage or early delivery. Sometimes it can cause the death of the healthy baby.

Prenatal management

Your doctor will probably want to monitor the pregnancy often with ultrasounds to see if the healthy baby is developing normally, and to watch out for early signs of heart failure.

If your doctor is concerned that the abnormal twin remnant is causing harm to the healthy baby, treatment before birth is sometimes needed. If treatment is needed, the most effective way to save the “pump” twin is to block all blood flow toward the acardiac “parasite” (which is not a live fetus, but a mass of tissue that grows because of blood and oxygen supplied by the healthy twin). Blocking the umbilical artery of the acardiac remnant effectively stops blood flow away from the healthy twin. Today, this is most often done with radiofrequency ablation (RFA). A special needle is inserted through the belly of the mother close to the umbilical cord of the twin remnant. A special instrument, connected to the needle, causes localized heat energy release, which closes off the blood flow to the acardiac twin remnant. This is not harmful to the mother or the healthy baby. This procedure is very similar to an amniocentesis, although the special needle is not as thin. While RFA is the least aggressive surgical treatment for severe TRAP, it is still considered an invasive procedure and should be done in a specialized center. Intervention is reserved for those cases where the life of the pump twin is in clear danger. Any intervention can be complicated by rupture of the membranes (PROM), premature delivery or even death of the healthy twin.

In pregnancies with TRAP, there is a greater risk that the healthy twin has a chromosomal defect, such as Down syndrome (about 1 in 20). Testing for this, usually by amniocentesis, may be offered, and the decision to have it or not is made by the parents.

What to expect at birth

Most women can have a normal birth and expect a healthy baby. If the twin remnant is small, it will often get pushed to the side and be delivered with the placenta. It may also shrink and be hard to see after delivery. If there is early labor, treatment at a hospital that can care for preterm newborn may be needed.

In some cases, the twin remnant is large and is closer to the cervix and may get in the way of a normal delivery. In that case, or if the healthy baby is breech (not head down), cesarean birth is the safer way to deliver the healthy twin.

After birth, the healthy baby usually won’t have any problems, unless delivery was early or there is a chromosomal defect. The twin remnant is not fully formed and can never survive.

Useful Links

Most Fetal Treatment Centers have useful information online. A list of NAFTNet centers, links and their respective website can be found by clicking here. 

Other useful sites, including national organizations, patient- and parent support groups, professional societies and governmental agencies, can be found on our resources page.  Be aware that, by clicking on these links, you will leave the NAFTNet site. NAFTNet does not endorse the content of these individual web sites.

Below are a few sites that have specific information on TRAP:

Images of TRAP sequence

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