In brief — the direct answer
Twin reversed arterial perfusion (TRAP) sequence is a complication of a shared-placenta pregnancy in which a pump twin supplies an acardiac twin through abnormal vessel connections. This can strain the pump twin’s circulation. Risk varies with the findings and progression; specialist assessment determines whether close monitoring or an intervention is appropriate.
What is TRAP Sequence (Acardiac Twin)?
TRAP is not a chromosomal issue but a problem of early placental development. Large blood vessel connections allow the pump twin to push blood 'backwards' into the acardiac twin. Because the acardiac twin receives blood that has already been deoxygenated by the pump twin, its upper body and heart do not develop. The acardiac twin exists as a mass of tissue and is not viable (it cannot survive outside the womb).
Who is at risk?
TRAP only occurs in Monochorionic (shared-placenta) twin pregnancies. It is usually discovered at the early 12-week or 16-week scans. It is a random event of pregnancy and not related to the mother's health or activities.
How we diagnose it
Diagnosis is made during a detailed ultrasound:
- Acardiac Presentation: One twin is seen with no heart activity and often has a poorly defined head, arms, and chest.
- Reversed Flow: Using color Doppler, we see blood flowing *towards* the acardiac twin through the umbilical artery, which is the opposite of normal.
- Pump Twin Monitoring: We look for signs of heart strain (hydrops), such as fluid around the heart or in the belly of the healthy twin.
What does management involve?
The goal is to stop the blood flow to the non-viable acardiac twin to save the healthy pump twin:
- Radiofrequency Ablation (RFA): A specialized needle is inserted under ultrasound guidance into the umbilical cord of the acardiac twin. We use heat (radiofrequency) to seal the blood vessels, effectively 'separating' the pump twin's circulation.
- Laser Ablation: In some cases, a laser may be used to achieve the same result.
- Observation: If the acardiac twin is very small and there are no signs of strain on the pump twin, we may monitor with weekly scans instead of immediate surgery.
What are the outcomes?
The potential benefit and risk depend on the diagnosis, gestation, technique and condition of the other twin. A procedure does not guarantee survival or freedom from neurological injury. Discuss the possibilities of losing one or both twins, membrane rupture, prematurity and further intervention, together with the alternatives. Published results from selected series must not be presented as this unit’s audited outcomes. Ongoing ultrasound and appropriate postnatal follow-up remain important.
When to see a subspecialist
If you are told one of your twins 'has no heart' or 'is not growing normally,' you need an urgent review by an MFM specialist familiar with TRAP.
Led by Dr. Werner Diehl, our center is one of the few in the region providing advanced RFA for TRAP sequence. We act quickly to protect the healthy twin and ensure the best possible outcome for your pregnancy.