What Is Twin Reversed Arterial Perfusion (TRAP) Sequence?

Discovering that you’re facing a TRAP (twin reversed arterial perfusion) sequence pregnancy can be an overwhelming and emotional experience. This isn’t what you expected when you learned you were pregnant with twins.

Gaining a better understanding of what TRAP sequence is, how it affects you and your babies, and your options for treatment can offer a sense of clarity and direction as you move through your pregnancy toward delivery.

In this article, we’ll walk through the essentials of TRAP sequence, including: 

  • What it is
  • How it’s diagnosed
  • Treatment approaches and intervention options
  • Common misconceptions 

While this path wasn’t what you expected, and it may feel daunting, remember you’re not in it alone. Your fetal care team is here to offer support, answers and guidance every step of the way. 

What Is TRAP Sequence? 

TRAP sequence is a rare and complex condition that affects only about one in every 35,000 to 50,000 pregnancies. It develops in about 1% of pregnancies where identical twins share a placenta (monochorionic twins). TRAP is considered a high-risk condition, and if diagnosed, you’ll be monitored closely by a maternal-fetal medicine team.¹

In TRAP sequence, one twin, the “pump twin,” develops normally while the other, known as the “acardiac twin,” does not.

The twins are connected by a shared blood vessel that runs through the placenta between their umbilical cords. The pump twin’s body has to work extra hard to provide blood to the acardiac twin.

While the acardiac twin receives blood from the pump twin, it does not develop a functional heart or other vital systems and cannot thrive. In reality, the acardiac twin (acardiac means without a heart) only grows because of blood supplied by the pump twin.² It’s difficult, but important, to understand that the acardiac twin cannot survive outside the womb.

This situation places extra demand on the pump twin’s heart, creating a risk of heart failure.³ TRAP sequence can also lead to other complications4 for the mother and pump twin, including the possibility of preterm labor, fluid buildup in the body and, in some cases, pregnancy loss or stillbirth.

What Causes TRAP Sequence? 

We don’t know exactly why TRAP sequence happens, but it’s important to know that there is nothing you did to cause this condition to develop.

No lifestyle factors or medications caused it, and it is not thought to be genetic. TRAP sequence is a random occurrence that is believed to result from changing blood pressure and blood flow during early pregnancy. Because there is no genetic component, one TRAP sequence pregnancy does not put you at higher risk in subsequent pregnancies. 

How Is TRAP Sequence Diagnosed? 

Diagnosing TRAP sequence typically starts with one of your regular prenatal ultrasounds. Your ultrasound will confirm that both twins share a placenta, and it may reveal that one twin is developing normally while the other shows unusual cardiac activity. Modern technology, including advanced 3D and 4D imaging, allows for the detection of this condition as early as 11 weeks.5

If your provider suspects TRAP sequence, the next step is usually a detailed fetal echocardiography. This is a specialized ultrasound that focuses on the heart and blood vessels and can assess blood flow between the twins, confirming the diagnosis.6 Depending on the situation, your provider may also recommend other tests, like a fetal MRI or amniocentesis. These tests provide your medical team with the insights they need to tailor care and treatment to your unique situation.

What Happens After a TRAP Sequence Diagnosis? 

Once you’ve been diagnosed with TRAP sequence, your medical team will focus on safeguarding your health and that of your developing baby, working towards the best possible outcome — a healthy mom and baby.

Your care will typically be coordinated by a maternal-fetal medicine specialist. These experts are trained to handle complex and high-risk pregnancies and will be your guide and support throughout your pregnancy.

The level of intervention in TRAP sequence pregnancies can vary. All TRAP sequence pregnancies require careful monitoring, but not all require intervention.

You’ll undergo frequent ultrasound and fetal echocardiography appointments throughout your pregnancy to monitor the healthy twin’s heart. If the team detects a risk of heart failure in the pump twin, your doctor may recommend fetal surgery to save the life of your otherwise healthy baby. 

TRAP Sequence Treatment Options

The goal of monitoring, intervention and treatment is to safeguard the life of the healthy twin. Part of this process involves assessing the size difference between the twins. As the acardiac twin grows larger, the pump twin’s heart has to work harder. If the acardiac twin grows to more than 75% of the pump twin’s weight, it can pose a significant risk for the growing baby, including the possibility of losing the pregnancy.7


Your fetal care team will consider all this and more when assessing whether intervention is appropriate. If treatment is needed, maternal-fetal medicine specialists have options to intervene, including: 

  • Delivery of the healthy twin. If you’re far enough along in your pregnancy, you may be able to deliver your healthy twin without the need for additional treatment. 
  • Radiofrequency ablation (RFA). For situations where the pump twin is at risk but not ready for delivery, RFA can be a lifesaving option. This minimally invasive procedure stops the flow of blood to the acardiac twin, allowing the healthy twin to continue developing.8
  • Other methods. There are also other surgical techniques available, such as inserting coils or alcohol into the umbilical cord, fetoscopic cord ligation, and laser or cautery methods.9 These aim to disrupt the blood flow to the acardiac twin, decreasing the risk for the pump twin.10

Throughout treatment and intervention, your fetal care team will be there to provide support and answer questions to ensure you’re comfortable with your care plan. 

Common Misconceptions About TRAP Sequence

It’s natural to encounter misinformation about rare conditions like TRAP sequence. Here are a few misconceptions about the condition to help you understand it better. 

Misconception: TRAP sequence can happen in any identical twin pregnancy. 

Fact: TRAP sequence occurs in monochorionic twins or those that share a placenta. While up to 70% of identical twins can share a placenta, only a small percentage (1%) of these develop TRAP sequence. Monochorionic twins may face other risks, including twin-to-twin transfusion syndrome (TTTS) and unequal placenta sharing.11 

Misconception: All cases of TRAP sequence require treatment. 

Fact: All cases of TRAP sequence require careful monitoring, but in some cases, the pump twin can grow and thrive without intervention. You’ll work closely with your care team to determine the best course of action after a TRAP sequence diagnosis. 

Misconception: The acardiac twin can survive. 

Fact: There is no chance in TRAP sequence that the acardiac twin can survive outside the womb. Acardiac means the twin lacks a heart and often does not develop an upper body. The acardiac twin is more akin to a mass of tissue than a developing fetus. This can be a tough reality to face, but understanding it can help you manage expectations as you navigate your treatment. 

Giving Birth After a TRAP Sequence Diagnosis

When it comes time to give birth to your baby, you’ll be assisted by your maternal-fetal medicine care team, including a neonatologist. About half of babies born with TRAP sequence will need intensive care immediately after birth. This could be for reasons such as heart problems, enlarged liver or spleen, fluid in the abdomen, swelling, or low levels of protein in the liver. If the baby was born prematurely, they may need help breathing as well.12

Despite these challenges, the outlook is most often positive. In the absence of additional complications, the pump twin has a very high chance — more than 90% — of a healthy outcome.13

This statistic can serve as a beacon of hope as you navigate your TRAP pregnancy and work with your care team to map out the best possible care plan for you and your baby.



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