<unk>  – 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<unk> . 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<unk> ing, 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.<unk>  share a single common chorion, placenta, and amniotic sac, although these characteristics are not exclusive to<unk> , 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<unk> .
Conjoined twins
Locked twins
Monochorionic twins
Monoamniotic twins
Vanishing twin
Parasitic twin
Craniopagus twins
Vestigial twin