Conjoined twins – sometimes popularly referred to as Siamese twins – are twins 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. Approximately half are stillborn, and an additional one-third die within 24 hours. Most live births are female, with a ratio of 3:1.Two 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 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.Chang 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. 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. Due to the brothers fame and the rarity of the condition, the term "Siamese twins" came to be associated with conjoined twins. Causes
There are two theories about the development of conjoined twins.
They are the earliest set of conjoined twins whose names are (purportedly) known. Lazarus and Joannes Baptista Colloredo (1617 — after 1646), autosite-and-parasite pair
Helen and Judith of Szony (Hungary, 1701 — 1723), pygopagus. Chang and Eng Bunker (1811–1874). The Bunker twins were born of Chinese origin in Siam (now  Thailand), and the expression Siamese twins is derived from their case. They were joined by the areas around their xiphoid cartilages, but over time, the connective tissue stretched. In 1834, a set of conjoined triplets were born in Cattania. Two of the heads shared a neck while the other head had its own. The infant, a male, was described by Galvagni. Millie and Christine McCoy (July 11, 1851 – October 8, 1912), (oblique pygopagus). The McCoy twins were born into slavery in Columbus County, North Carolina, United States. They went by the stage names "The Two-Headed Nightingale" and "The Eighth Wonder of the World" and had an extensive career before retiring to the farm on which they were born. Giacomo and Giovanni Battista Tocci (1875? — 1912? ), (dicephalus tetrabrachius dipus)
Josefa and Rosa Blazek (January 20, 1878 — March 30, 1922), pygopagus. The Blazek twins were born in Skrejšov, Bohemia (now the Czech Republic). They began performing in public exhibitions at the age of 13, and their act later included Rosas son Franz. The sisters died in Chicago, Illinois. Born 20th century
Daisy and Violet Hilton of Brighton, England (1908–1969), pygopagus. The Hilton twins were performers who played musical instruments, sang, and danced.