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monozygotic twins whose bodies are joined. The connection may be slight or extensive; it may involve skin and muscles or cartilage of a limited region, such as the head, chest, hip, or buttock, or the twins may share an organ, such as an intestine or parts of the spine. If joined superficially, the twins may be easily separated by surgery soon after birth. If more deeply united, they may have to go through life with their handicap, if they survive. New techniques in surgery are making it possible to separate some Siamese twins whose physical links are highly complex. Called also conjoined twins
originally, a much-publicized conjoined pair of twins (xiphopagus) born in Siam (present-day Thailand) in the 19th century; this term has since come into general lay usage for any type of conjoined twins, but is incorrect.
Prognosis Depends on the need for or adequacy of surgical separation
'Siamese' twinsConjoined equal twins A joined gestational product due to a failure in division of the yolk sac or due to delayed monovular separation; STs occur in ±1:200,000 term deliveries; most are joined at the chest–thoracopagus Prognosis Depends on adequacy of surgical separation
Si·a·mese twins(sī'ă-mēz' twinz)
A much publicized pair of conjoined twins born in Thailand (then Siam) in the 19th century; this term has since come into general lay usage for any type of conjoined twins.