Anomalous twin [twin reversed arterial perfusion (TRAP) twin] perfused by deoxygenated blood from pump twin

Blood enters fetus via umbilical artery (UA)

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  Reversed perfusion → selective development of torso/lower extremities

  
  
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  Lack of umbilical vein flow into heart → impaired/absent cardiac development

Must be monochorionic gestation

Flow in UA of anomalous twin is toward fetus

TRAP twin anomalies are lethal

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  Dysmorphic fetus with edema and cyst formation in soft tissues

  
  
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  Usually recognizable torso and lower extremities

  
  
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  Rudimentary heart may exist

  
  
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  Often no identifiable cranial structures

  
  
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  Presence of upper extremities variable

You will never miss this diagnosis if you check direction of UA flow in anomalous twins

Careful search for anomalies of pump twin as intervention only indicated to salvage healthy pump twin

Compare abdominal circumferences: Prognosis worse if ≥ 50% difference

Monitor size of abnormal twin

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  Prolate ellipsoid formula (width x height x length x 0.523) most accurate

  
  
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  TRAP twin > 70% of pump twin → increased risk of pump twin compromise

Monitor fluid volume: Polyhydramnios is poor prognostic indicator

Look for signs of impending hydrops in pump twin

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  Tricuspid regurgitation

  
  
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  Abnormal flow in ductus venosus