TRAP sequence in pregnancy refers to a rare, serious complication in identical twin pregnancies, not a general “trap” or social situation.

What is TRAP sequence?

Twin Reversed Arterial Perfusion (TRAP) sequence happens only in monochorionic (identical) twin pregnancies where the twins share one placenta.

In this condition, one twin develops without a functioning heart (the acardiac twin) and survives only because it receives blood from the other, structurally normal “pump” twin.

Because the pump twin has to circulate blood for both fetuses, its heart is under heavy strain and the pregnancy is considered high risk.

How common is it?

TRAP sequence is rare, occurring in about 1 in 35,000 pregnancies and in less than 1% of identical twin pregnancies.

Even though it is rare overall, any monochorionic twin pregnancy is screened more closely because several serious complications, including TRAP, are more likely.

Why is it dangerous?

The main danger is to the pump twin, whose life is actually at risk.

Major complications can include:

  • Heart failure in the pump twin due to the extra workload.
  • Hydrops (severe fluid buildup) in the pump twin, which is life-threatening.
  • Excess amniotic fluid (polyhydramnios or hydramnios), which increases the chance of preterm labor and maternal discomfort.
  • Miscarriage, stillbirth, or death of the pump twin shortly after birth if not managed appropriately.

The larger the acardiac twin grows, the higher the risk of heart failure and poor outcome for the pump twin.

How is TRAP sequence diagnosed?

TRAP sequence is typically picked up on prenatal ultrasound.

Key ultrasound features include:

  • Twins that share a single placenta (monochorionic pregnancy).
  • One fetus with normal growth and visible heart activity (pump twin).
  • One fetus with absent heart activity and severe malformations, often with poorly formed upper body (acardiac twin).
  • Often, significantly increased amniotic fluid around the pump twin.

With advanced imaging such as 3‑D and 4‑D ultrasound, TRAP can sometimes be diagnosed as early as around 11 weeks of gestation.

Fetal echocardiography is often used to evaluate the pump twin’s heart function and detect signs of strain or heart failure.

Other tests, like amniocentesis, may be used to look for additional genetic issues if the care team feels it is necessary.

What does management and treatment look like?

Management depends on how severely the pump twin is affected and how large and active the acardiac twin is.

Common elements of care include:

  • Close monitoring
    • Frequent ultrasounds to track twin growth, amniotic fluid, and blood flow.
* Detailed Doppler studies to assess circulation between the twins.

* Ongoing assessment of the pump twin’s heart function and any signs of hydrops.
  • Expectant (watchful) management
    • Used when the acardiac twin is relatively small and the pump twin shows no signs of heart failure.
* The pregnancy is closely followed, and delivery is timed to balance maturity with the risk of sudden deterioration.
  • Fetal surgery / cord occlusion
    • In higher‑risk cases—such as a very large acardiac twin, polyhydramnios, or signs of heart failure in the pump twin—interventions may be offered to stop blood flow to the acardiac twin.
* Techniques include umbilical cord occlusion or other methods to interrupt the abnormal circulation.

* These procedures aim to protect the pump twin by reducing cardiac workload and stabilizing the pregnancy.

Some centers report very high survival rates (near 98%) for the pump twin with appropriate treatment, with average delivery around 35 weeks in higher‑risk treated cases.

Without intervention in clearly high‑risk situations (large acardiac twin, polyhydramnios, heart strain), the risk of pregnancy loss or extreme prematurity can be substantial—on the order of 50–75% in some expectantly managed cohorts.

Outlook and key takeaways

Although TRAP sequence has a serious reputation, outcomes have improved significantly with early diagnosis, specialized monitoring, and, when indicated, fetal therapy.

For low‑risk cases (smaller acardiac twin, stable pump twin, controlled fluid levels), term or near‑term delivery and good outcomes are increasingly common.

If you or someone you know has been told there may be TRAP sequence in a pregnancy, the most important steps are:

  1. Confirm care with a maternal–fetal medicine specialist or fetal therapy center experienced in monochorionic twin complications.
  1. Ensure regular high‑level ultrasounds and fetal echocardiography as recommended.
  1. Discuss all management options (watchful waiting, possible fetal intervention, delivery planning) and their risks and benefits.

Information gathered from public forums or data available on the internet and portrayed here.