TRAP Sequence (Acardiac Twin)

TRAP is not a standard twin pregnancy.

• The Acardiac Twin: Is effectively a mass of tissues and limbs that grows because it is receiving blood from its sibling. It is not 'alive' in the conventional sense.
• The Pump Twin: Is a completely normal, healthy baby who is at high risk due to the 'parasitic' nature of the blood transfusion. The more the acardiac twin grows, the harder the pump twin's heart has to work.

Without intervention, the risk of losing the healthy pump twin is over 50-70%.

TRAP sequence is extremely rare, affecting approximately 1 in 35,000 pregnancies or about 1% of all monochorionic (MC) twins.

Because all MC twins share a placenta, they are all technically at risk, but TRAP is usually identified very early—often during the 12-week nuchal translucency scan or even earlier—because one twin looks significantly malformed and lacks a heartbeat.

At the MFM Unit, we use high-resolution Doppler to confirm the diagnosis:

1. Doppler Confirmation: We see 'reversed' blood flow in the umbilical cord of the acardiac twin (blood flowing *into* the baby rather than out).
2. Acardiac/Pump Weight Ratio: We measure the size of the acardiac twin relative to the healthy pump twin. If the acardiac mass is more than 70% of the pump twin's weight, the risk of heart failure is extreme.
3. Pump Heart Assessment: Looking for signs of heart enlargement or fluid around the pump twin's heart.

The goal of management is to save the healthy pump twin. This is achieved by stopping the blood flow to the acardiac twin:

• Radiofrequency Ablation (RFA) or Laser Coagulation: Between 16 and 19 weeks (or later if needed), a tiny needle or laser fiber is inserted into the umbilical cord of the acardiac twin. Using heat (RFA) or light (Laser), we seal off the blood vessels. This immediately stops the strain on the pump twin.
• The Result: Once the connection is closed, the acardiac twin stops growing, and the pump twin's heart can return to its normal work level. The pump twin then continues to grow as a 'singleton' for the rest of the pregnancy.

With modern fetal intervention performed by experts like Dr. Werner Diehl:

• The survival rate for the healthy pump twin is approximately 90-95%.
• Most pump twins can be delivered near term (37 weeks) and do not have any long-term health issues related to the TRAP sequence.
• Early intervention is key; if we wait until the pump twin is already in heart failure, the success rate is lower.

If you have a twin pregnancy and were told that 'one twin has no heartbeat but is still growing' or that 'the blood is flowing backward,' you must be referred to a specialized fetal therapy center immediately.

TRAP sequence is a specialized surgical condition. Our Unit provides the advanced RFA and laser technologies required to protect the healthy twin and ensure the best possible outcome for your pregnancy.