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Twin reversed arterial perfusion is a rare complication of monochorionic 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. It can result in TRAP syndrome, causing mortality and deformities in both twins. ==Acardiac twin== 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 (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 perfusion 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. (1) Acephalus - The most common type, lacking a head, though it may have arms. Thoracic organs are generally absent, and disorganized & unidentifiable tissues take their place. (2) Anceps - The acardius has most body parts, including a head with face and incomplete brain. Organs, though present, are crudely formed. (3) Acormus - This type has no apparent body and the umbilical cord is seemingly attached to the neck, but x-rays or dissection reveal thoracic structures in the apparent head. One had a leg apparently attached to the head. This may be due to embryopathy degenerating a once normal embryo. (4) Amorphus - This extreme form not only lacks a head and limbs, but also any internal organs, and consists of tissues with blood vessels branching from the umbilical cord. Some may only be stem cell tumors. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Twin reversed arterial perfusion」の詳細全文を読む スポンサード リンク
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