Monozygotic Monoamniotic Twin Pregnancy with TRAP Sequence and Acardiac Amorphous Foetus.

ABSTRACT

The ‘Twin Reversed Arterial Perfusion (TRAP)’ sequence or acardiac twinning, or chorioangiopagus parasiticus is an anomaly which is unique to the monochorionic multiple pregnancies and is a rare complication that affects around 1 in 100 monozygotic twin pregnancies and about 1 in 35,000 overall, with no recognized familial recurrence.[1] They have almost doubled over the last two decades as a result of both the increasing average maternal age and with the increased use of assisted reproductive techniques. Physiologically, there is lack of a well-formed cardiac structure in one foetus (acardiac) that acts as a parasite as it is haemodynamically dependent on the structurally normal co-twin (pump twin) through a superficial artery-to-artery placental anastomosis. Various theories that explain the pathogenesis of the anomaly attribute it to either dysmorphogenesis or presence of arterio-arterial anastomosis, with latter being the most popular.[2] In arterio-arterial anastomosis there is a reversal of blood flow from the umbilical artery of pump twin into umbilical artery of the perfused twin therefore bypassing the placenta, henceforth poorly oxygenated blood contributes to deficient development of the head, upper limbs, torso and heart while the better oxygenated blood flowing via umbilical artery through common iliac artery and aorta allows better development of the lower part of the body.[1]-[3]