Conjoined twins or Siamese twins are identical twins^[1] joined in utero. 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.^[2] Approximately half are stillborn, and an additional one-third die within 24 hours. Most live births are female, with a ratio of 3:1.^[2][3]

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.^[4] The other theory, no longer believed to be the basis of conjoined twinning,^[4] 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. 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.^[5]

Chang and Eng Bunker (1811–1874) were brothers born in Siam who traveled widely for many years and were labeled as The Siamese Twins. Chang and Eng were joined at the torso by a band of flesh, cartilage, and their fused livers. In modern times, they could have been easily separated.^[6] 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.

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