Acephaly

Twin Reversed Arterial Perfusion sequence—also called TRAP sequence, TRAPS, or acardiac twinning—is a rare complication of monochorionic twin pregnancies. It is a severe variant of twin-to-twin transfusion syndrome (TTTS). The twins' blood systems are connected instead of independent. One twin, called the acardiac twin or TRAP fetus, is severely malformed. The heart is missing or deformed, hence the name acardiac, as are the upper structures of the body . The legs may be partially present or missing, and internal structures of the torso are often poorly formed. The other twin is usually normal in appearance. The normal twin, called the pump twin, drives blood through both fetuses. It is called reversed arterial perfusion because in the acardiac twin the blood flows in a reversed direction.

TRAP sequence occurs in 1% of monochorionic twin pregnancies and in 1 in 35,000 pregnancies overall.

The acardiac twin is a parasitic twin that fails to properly develop a heart, and therefore generally does not develop the upper structures of the body. 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 (which often only has 2 blood vessels, instead of 3), 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 oxygenation it is unable to develop the structures necessary for life, and presents with dramatic deformities.

Although no two acardiac twins are alike, twins with this disorder are grouped into 4 classes: Acephalus, anceps, acormus, and amorphus.

The acardiac twin may also be described as a hemiacardius, which has an incompletely formed heart, or a holoacardius, in which the heart is not present at all.

Generally the pump twin is structurally normal, although it is smaller than normal, but due to 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 if left untreated. The rate of fatality depends on the relative size of the acardiac twin. If the abnormal twin is greater than 50% of the size of the pump twin, the survival rate for the pump twin is only 10%.

...
Wikipedia