Twin reversed arterial perfusion is a rare complication of monochorionic[1] twin pregnancies, involving an acardiac twin whose structural defects are incompatible with life, and an otherwise normal "pump" twin. It occurs in 1% of monochorionic twin pregnancies and in 1 in 35,000 pregnancies overall.[2] It can result in TRAP syndrome, causing mortality and deformities in both twins.
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The acardiac twin is a parasitic twin that fails to develop a head, arms and a heart. The parasitic twin, little more than a torso with or without legs, receives its blood supply from the host twin by means of an umbilical cord-like structure, much like a fetus in fetu, except the acardiac twin is outside the host twin's body. Although the reason is not fully understood, it is apparent that deoxygenated blood from the pump twin is perfused to the acardiac twin. The acardiac twin grows along with the pump twin, but due to inadequate perfusion it is unable to develop the structures necessary for life, and presents with dramatic deformities.
Structurally the pump twin is generally normal, but due some of the related problems including the rapid growth of the acardiac twin, polyhydramnios, and heart failure due to high output, there is a high mortality rate for the pump twin.
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