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Thoraco-omphalopagus (28% of cases): Two bodies fused from the upper chest to the lower chest.

The most common types of conjoined twins are: Types Ĭonjoined twins are typically classified by the point at which their bodies are joined. Over- or underexpression of this gene in laboratory animals results in severe malformations of the head region, including duplications, similar to some types of conjoined twins. Goosecoid activates inhibitors of BMP4 and contributes to regulation of head development. Misexpression of genes, such as Goosecoid, may also result in conjoined twins. The type of twins formed depends on when and to what extent abnormalities of the node and streak occurred. Occasionally, monozygotic twins are connected only by a common skin bridge or by a common liver bridge. These twins are classified according to the nature and degree of their union.

Partial splitting of the primitive node and streak may result in the formation of conjoined twins. The second theory is that a fusion of two fertilized eggs occurs earlier in development. If the zygote division occurs after two weeks of the development of the embryonic disc, it results in the formation of conjoined twins.

The first is that a single fertilized egg does not fully split during the process of forming identical twins. There are two theories about the development of conjoined twins. Due to the brothers' fame and the rarity of the condition, the term "Siamese twins" came to be associated with conjoined twins. In modern times, they could have been easily separated. Chang and Eng were joined at the torso by a band of flesh, cartilage, and their fused livers. Ĭhang and Eng Bunker (1811–1874) were brothers born in Siam (now Thailand) who traveled widely for many years and were labeled as The Siamese Twins. 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 who also share these structures in utero. 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 similar stem cells on the other twin and fuse the twins together. The more generally accepted theory is fission, in which the fertilized egg splits partially. Two theories exist to explain the origins of conjoined twins.

Most live births are female, with a ratio of 3:1. Approximately half are stillborn, and an additional one-third die within 24 hours. A very 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 Southwest Asia and Africa. X-ray of conjoined twins, Cephalothoracopagus.ĭepends on type Occasionally may surviveĬonjoined twins – sometimes popularly referred to as Siamese twins – are twins joined in utero.