The other explanation, no longer believed to be accurate, is fusion, in which the fertilized egg completely separates, but stem cells (that search for similar cells) find similar stem cells on the other twin and fuse the twins together. The one that is generally accepted is fission, in which the fertilized egg splits partially. Two possible explanations of the cause of conjoined twins have been proposed. Most live births are female, with a ratio of 3:1. Approximately half are stillborn, and an additional one-third die within 24 hours. It is a very rare phenomenon, estimated to occur in anywhere between one in 49,000 births to one 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 – popularly referred to as Siamese twins – are twins joined in utero.
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