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Diagnosis and management of twin reversed arterial perfusion (TRAP) sequence

Joan M Mastrobattista, MD
Michael J Lucas, MD, MPH
Section Editors
Lynn L Simpson, MD
Deborah Levine, MD
Deputy Editor
Vanessa A Barss, MD, FACOG


Twin reversed arterial perfusion sequence (TRAP) refers to a rare, unique complication of monochorionic twin pregnancy in which a twin with an absent or a nonfunctioning heart ("acardiac twin") is perfused by its co-twin ("pump twin") via placental arterial anastomoses. The acardiac twin usually has a poorly developed heart, upper body, and head. The pump twin is at risk of heart failure and problems related to preterm birth.


TRAP sequence has historically been reported to occur in about 1 percent of monochorionic twin pregnancies and 1 in 35,000 pregnancies [1]. These figures are widely cited but based on data available up to 1953. In contemporary obstetrics, the incidence appears to be much higher when factors such as the use of first trimester obstetric ultrasound examination, which detects twin demises early in gestation, and assisted reproductive techniques, which have increased the incidence of twins including monochorionic twins, are accounted for. A 2015 study estimated the incidence of acardiac twins is 2.6 percent of monochorionic twin pregnancies and 1 in 9500 to 11,000 pregnancies [2].


In the normal fetal circulation, blood from the placenta flows through the umbilical vein to the fetus. From there, the ductus venosus shunts 80 percent of the placental blood flow into the inferior vena cava, where it mixes with venous return from the lower extremities and kidneys before entering the right atrium (figure 1). Once the blood enters the right atrium, the two sides of the fetal heart act in parallel through intra- and extra-cardiac shunts (foramen ovale, ductus arteriosus) to fill the aorta and provide systemic circulation. The distal aorta terminates in the left and right common iliac arteries, which each divide into internal and external iliac branches. The umbilical arteries carry blood from the internal iliac arteries back to the placenta.

In TRAP sequence, the pump twin maintains this normal pattern of fetal circulation. In addition, a portion of its cardiac output travels through placental arterial-arterial anastomoses to the umbilical artery and eventually to the systemic circulation of the recipient co-twin, thus creating "reversed" circulation in this twin. This is possible because the acardiac twin lacks a functional heart, whose pumping would normally provide forward flow and high systemic pressure. The presence of arterial-arterial anastomoses allows blood to be pumped from the normal twin to the acardiac twin without passing through a capillary bed. Veno-venous and arterio-venous anastomoses also occur. The presence of placental vascular anastomoses is common in monochorionic twins and alone is not sufficient for the development of TRAP sequence.

Unequal vascular perfusion from the pump twin results in the evolution of a variety of structural abnormalities in the recipient twin [3]. The abnormal circulatory pattern provides perfusion of mixed or medium oxygenated blood from the pump twin to the lower half of the recipient twin via one of its iliac arteries, but poor perfusion of the upper torso and head, which are more distal. Tissue necrosis of the more distal parts of the recipient occlude capillaries, with no mechanism for circulation back into the venous system. The arterial circulation into the lower extremities returns backwards to the bifurcation of the aorta and then forward into the opposite iliac arteries and finally into the cord to return to the pump twin. Venous return from perfused tissues contributes to "growth" of the amorphous upper body mass, as there is no mechanism for return to the pump twin once the inferior vena cava and ductus venosus have occluded.

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Literature review current through: Nov 2017. | This topic last updated: Jul 19, 2017.
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