A rare complication of monochorionic twin pregnancy: Twin-reversed arterial perfusion (TRAP) sequence

Abstract

The twin-reversed arterial perfusion (TRAP) sequence, also known as acardiac twinning, is an anomaly unique to monochorionic multiple pregnancies that affects approximately 1 of 35,000 pregnancies and 1% of monochorionic twins.1 In TRAP, blood flows from an umbilical artery of the pump twin in a reverse direction into the umbilical artery of the perfused twin, via an arterioarterial anastomosis. The perfused (or acardiac) twin is a true parasite. Its blood is poorly oxygenated, and the hypoxemia contributes to variable degrees of deficient development of the head, heart, and upper limb structures. The lower half of the body is usually better developed, which may be explained by the mechanism of transfusion of the acardiac twin. Failure of head growth is called acardius acephalus; a partially developed head with identifiable limbs is called acardius myelacephalus; and failure of any recognizable structure to form is acardius amorphous. We report a case of acardius amorphous.