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Conjoined twins are identical twins〔(【引用サイトリンク】url=http://www.umm.edu/conjoined_twins/facts.htm )〕 joined in utero. An extremely rare phenomenon, the occurrence is estimated to range from 1 in 49,000 births to 1 in 189,000 births, with a somewhat higher incidence in Southeast Asia, Africa and Brazil. Approximately half are stillborn, and an additional one-third die within 24 hours. Most live births are female, with a ratio of 3:1. Most stillborns are male.〔〔(【引用サイトリンク】url=http://emedicine.medscape.com/article/934680-overview ) (this article includes post-mortem images)〕 Two contradicting theories exist to explain the origins of conjoined twins. The more generally accepted theory is ''fission'', in which the fertilized egg splits partially. The other theory, no longer believed to be the basis of conjoined twinning,〔 is fusion, in which a fertilized egg completely separates, but stem cells (which search for similar cells) find like-stem cells on the other twin and fuse the twins together. Conjoined twins share a single common chorion, placenta, and amniotic sac, although these characteristics are not exclusive to conjoined twins as there are some monozygotic but non-conjoined twins that also share these structures in utero.〔 〕 The most famous pair of conjoined twins was Chang and Eng Bunker (Thai: อิน-จัน, In-Chan) (1811–1874), Thai brothers born in Siam, now Thailand. They travelled with P.T. Barnum's circus for many years and were labeled as the Siamese Twins. Chang and Eng were joined by a band of flesh, cartilage, and their fused livers at the torso. In modern times, they could have been easily separated. Due to the brothers' fame and the rarity of the condition, the term "Siamese twins" came to be used as a synonym for conjoined twins. == Types of conjoined twins == Conjoined twins are typically classified by the point at which their bodies are joined. The most common types of conjoined twins are: * Thoraco-omphalopagus (28% of cases):〔 Two bodies fused from the upper chest to the lower chest. These twins usually share a heart, and may also share the liver or part of the digestive system.〔() 〕 * Thoracopagus (18.5%):〔 Two bodies fused from the upper thorax to lower belly. The heart is always involved in these cases.〔 As of 2015, separation of a genuinely shared heart has not offered survival to two twins; a designated twin may survive if allotted the heart, sacrificing the other twin. * Omphalopagus (10%):〔 Two bodies fused at the lower abdomen. Unlike thoracopagus, the heart is never involved in these cases; however, the twins often share a liver, digestive system, diaphragm and other organs.〔 * Parasitic twins (10%):〔 Twins that are asymmetrically conjoined, resulting in one twin that is small, less formed, and dependent on the larger twin for survival. * Craniopagus (6%):〔 Fused skulls, but separate bodies. These twins can be conjoined at the back of the head, the front of the head, or the side of the head, but not on the face or the base of the skull.〔 Other less-common types of conjoined twins include: * Cephalopagus: Two faces on opposite sides of a single, conjoined head; the upper portion of the body is fused while the bottom portions are separate. These twins generally cannot survive due to severe malformations of the brain. Also known as janiceps (after the two-faced god Janus) or syncephalus.〔 * Syncephalus: One head with a single face but four ears, and two bodies.〔 * Cephalothoracopagus: Bodies fused in the head and thorax. In this type of twins, there are two faces facing in opposite directions, or sometimes a single face and an enlarged skull.〔 * Xiphopagus: Two bodies fused in the xiphoid cartilage, which is approximately from the navel to the lower breastbone. These twins almost never share any vital organs, with the exception of the liver.〔 A famous example is Chang and Eng Bunker. * Ischiopagus: Fused lower half of the two bodies, with spines conjoined end-to-end at a 180° angle. These twins have four arms; two, three or four legs; and typically one external set of genitalia and anus.〔 * Omphalo-Ischiopagus: Fused in a similar fashion as ischiopagus twins, but facing each other with a joined abdomen akin to omphalopagus. These twins have four arms, and two, three, or four legs.〔 * Parapagus: Fused side-by-side with a shared pelvis. Twins that are dithoracic parapagus are fused at the abdomen and pelvis, but not the thorax. Twins that are diprosopic parapagus have one trunk and two faces. Twins that are dicephalic parapagus are dicephalic, and have two (dibrachius), three (tribrachius), or four (tetrabrachius) arms.〔 * Craniopagus parasiticus: Like craniopagus, but with a second bodiless head attached to the dominant head. * Pygopagus (Iliopagus): Two bodies joined at the pelvis.〔 * Rachipagus: Twins joined along the dorsal aspect (back) of their bodies, with fusion of the vertebral arches and the soft tissue from the head to the buttocks 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「Conjoined twins」の詳細全文を読む スポンサード リンク
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