Twin reversed arterial perfusion (TRAP) sequence

Denise McGrath1,2, S Briody¹ N Ravikumar¹, M Moran², ¹Fetal Assessment Unit, Regional Hospital Mullingar, Mullingar, Westmeath, Ireland ²Radiography and Diagnostic Imaging, School of Medicine, University College Dublin, Ireland


Twin reversed arterial perfusion (TRAP) sequence, is where one monochorionic twin (acardiac) has an absent, rudimentary or non-functioning heart with a normal second (pump) twin. It is hypothesised that the acardiac twin has no direct vascular connection to the placenta, obtaining all of its blood supply through an arterio-arterial communication from the unaffected twin.

Case Report:

A case of twin reversed arterial perfusion (TRAP) sequence was diagnosed at 12 weeks’ gestation using transvaginal color Doppler ultrasound, which demonstrated the presence of retrograde perfusion in the umbilical artery of the abnormal twin. Ultrasound imaging showed a mono-chorionic–diamniotic twin pregnancy with an inappropriately grown second twin, the morphological evaluation of which revealed an abnormal cephalic pole with acrania, diffuse subcutaneous edema and the presence of cardiac activity in an abnormal heart with a single chamber.

We report a case of TRAP sequence diagnosed at 13 weeks gestation. Ultrasound diagnosed a monochorionic–monoamniotic twin pregnancy, the second twin inappropriately grown with acardiac amorphus. Colour Doppler displayed reversed arterial blood flow towards the acardiac twin via the umbilical artery. The pump twin delivered at term.


The pump twin has a high mortality rate due to cardiac failure. Early and accurate diagnosis facilitated appropriate referral for assessment and monitoring resulting in a good outcome for the pump twin.

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