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Key Points

  • Twin reversed arterial perfusion (TRAP) sequence, also known as acardia, is a rare anomaly unique to multiple gestation in which one twin has an absent, rudimentary, or nonfunctioning heart (acardiac twin) and the other twin is normal (pump twin). TRAP sequence has been associated with adverse perinatal outcomes.

  • The placentation in the majority of acradic twins is monochorionic diamniotic.

  • The fundamental requirement for the TRAP sequence is the development of arterial-to-arterial vascular anastomoses between the umbilical arteries of twins early in embryogenesis.

  • The diagnosis is made with ultrasound. The features useful in the diagnosis of acardia include absence of normal cardiac structure and cardiac movement and variable structural abnormalities.

  • The malformations found in cases of acardia include growth abnormalities, partial or complete absence of the cranial vault, anencephaly, holoprosencephaly, absent or rudimentary facial features, absent or rudimentary upper and/or lower limbs, absent lungs and heart, gastrointestinal atresia, omphalocele, gastroschisis, and absent liver, pancreas, spleen, and kidneys.

  • The pump twin is usually morphologically normal, and the risk of aneuploidy is 9%.

  • The goal of antepartum management of a pregnancy complicated by the TRAP sequence is to maximize outcome for the structurally normal pump twin.

  • In the absence of poor prognostic features (twin weight ratio >0.70, elevated combined ventricular output, elevated cardiothoracic ratio, congestive cardiac failure, polyhydramnios), expectant management with serial sonographic evaluation is reasonable.

  • Intrafetal radiofrequency ablation of the cord of the acardius when indicated by above criteria is associated with 95% pump twin survival.


Twin reversed arterial perfusion (TRAP) sequence, also known as acardia, is a rare anomaly unique to multiple gestation in which one twin has an absent, rudimentary, or non-functioning heart. Schatz (1898) classified acardia into two main groups: hemiacardius (imperfectly formed heart) and holoacardius (absence of heart). Das (1902) subdivided acar-dia into four groups: acardius acephalus, acardius amorphus or anideus, acardius acormus, and acardius anceps or para-cephalus (Table 120-1). Simonds and Gowen (1925) added a further subgroup: acardius myelacephalus.

Table Graphic Jump Location
Table 120-1Classification of the Conditions of Acardia

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