Conjoined twins are identical twins joined in uterus. A rare phenomenon, the occurrence is estimated 1 in 189,000 births, with a somewhat higher incidence in Southwest Asia, Africa and Brazil. Approximately half are stillborn, and a smaller fraction of pairs born alive have abnormalities incompatible with life. Siamese twins are formed from a single egg which develops into two almost separate balls of cells. In normal embryo and foetus development every cell knows where it is in the body because the neighbours produce chemical messages. So a skin cell knows not only it is skin, but that it is – say – nose skin, rather than chin or ear or lip skin. In Siamese twins these chemical messages don’t work properly. The end results can be very bizarre: a single organism with two heads, two hearts, four legs and arms – or is that single organism actually two people, two individual Siamese twins?
The most famous pair of conjoined twins was Chang and Eng Bunker (1815–1880), Thai brothers born in Siam, now Thailand. They travelled with P.T. Barnum’s circus for many years and were labeled as the Siamese Twins. Chang and Eng were joined by a band of flesh, cartilage, and their fused livers at the torso. 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 used as a synonym for conjoined twins.
Surgery to separate conjoined twins may range from very easy to very hard, depending on the point of attachment and the internal parts that are shared. Most cases of separation are extremely risky and life-threatening. In many cases, the surgery results in the death of one or both of the twins, particularly if they are joined at the head or share a vital organ. This makes the ethics of surgical separation, where the twins can survive if not separated, contentious. The first such successful separation was carried out on 17th September 1953, at the Foundation Hospital, New Orleons, USA. Carolyn Anne and catherine Anne Mouton were separated in a three hour long operation.