What is it?

Twin Reversed Arterial Perfusion (TRAP) sequence is a rare complication specific to monochorionic twins (twins sharing a single placenta). A severely deformed “acardiac” twin that often lacks a heart is supported by a co-twin that pumps blood to it across a shared placental artery. This places the “pump” twin at risk of heart failure, early delivery, and loss of life due to the extra work it must do to support the “acardiac” twin. Importantly, the “acardiac” twin has no chance for survival after delivery.

How common is it?

TRAP is an extremely rare complication, estimated to occur in approximately 1 in every 35,000 pregnancies.

How is it managed?

Evaluation of a pregnancy complicated by TRAP sequence requires ultrasound assessment of the size and anatomy of both “pump” and “acardiac” twins. Fetal echocardiography is recommended due to the association of this condition with fetal heart defects, as well as to assess heart function of the “pump” twin. Amniocentesis is also encouraged due to risk of serious chromosomal problems associated with this condition. In cases where the “acardiac” twin appears large or there is concern for “pump” twin heart failure, Radiofrequency Ablation (RFA) may be recommended. This procedure permanently interrupts blood flow across the artery connecting the twins and relieves the “pump” twin of the burden of supporting the severely abnormal “acardiac” fetus.


While RFA likely offers improved outcomes when compared to providing no treatment in more severe cases, there is little information regarding long-term outcomes of survivors of TRAP sequence. Prematurity is a potential concern with or without RFA therapy and this may also influence outcomes.