Twin Reversed Arterial Perfusion (TRAP · Acardiac Twin)

Step-by-step decision

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Full pathway

1. [Decision] Weight ratio + pump-twin cardiac function
   Acardiac/pump twin weight ratio + pump-twin cardiac function? (TRAP occurs only in monochorionic twins. One twin = an acardiac mass (absent/rudimentary heart, absent upper body/head) reverse-perfused by the pump twin through an artery-to-artery anastomosis (Doppler shows reversed flow in the acardiac twin). ~1% of monochorionic. Pump twin at risk of high-output heart failure, polyhydramnios, prematurity, hydrops, growth restriction. First exclude pump-twin chromosomal abnormality.)
   • Acardiac/pump twin weight ratio <70%, pump twin no heart failure → Ratio <70% · expectant weekly follow-up
   • Weight ratio >70% or pump-twin heart failure/hydrops/polyhydramnios → Ratio >70%/pump-twin involvement → RFA
2. [End] Ratio <70% · expectant weekly follow-up
   Weight ratio <70%, pump twin no heart failure: expectant + weekly ultrasound (monitor acardiac mass growth, pump-twin cardiac function/fluid/Doppler); progression (rapid growth/heart failure signs) → refer for intervention; multidisciplinary follow-up at a fetal center.
3. [End] Ratio >70%/pump-twin involvement → RFA
   Weight ratio >70% or pump-twin heart failure/hydrops/polyhydramnios: high pump-twin high-output heart failure risk (untreated mortality 50–75%); refer to a fetal therapy center for radiofrequency ablation (RFA, least invasive) or cord occlusion to interrupt the acardiac twin's blood supply (late first/early second trimester) to protect the pump twin; serial monitoring afterward.

Source guidelines & references

• TRAP sequence (acardiac twin) diagnosis and radiofrequency ablation (weight ratio 70%; RFA/cord occlusion)

This pathway is our own synthesis of the decision logic in the guidelines above (not the guideline verbatim); thresholds and workflows change as guidelines update — in practice follow the latest guideline, your institution's protocol and the individual patient.

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