Siamese twins, also known as conjoined twins, are identical twins that did not fully separate during early embryonic development. In most cases, identical twins are formed from a single fertilized egg that splits into two separate embryos very early in pregnancy. Siamese twins come about when the egg does not divide entirely, resulting in two infants that are fused together. This occurrence is fairly rare, with only one in 200,000 live births being conjoined twins.
The prognosis for Siamese twins depends on several factors. First, most conjoined twins share a placenta and amniotic sac during fetal development. In some cases, one twin may receive more nourishment than the other or the placenta may not be able to provide enough support for two infants at once. This can also happen with fully separated identical twins. For this reason, between 40 and 60 percent of all conjoined twins arrive stillborn.
Upon being born, Siamese twins face additional obstacles. Depending on where they are joined, many Siamese twins share vital organ systems. Both babies may have only one heart or liver between them, meaning the single organ has to support both infants at once. This can lead to tremendous strain on both babies as they grow because organs are not made to support two growing children at the same time. Many times, one or both babies will die within their first few years of life.
In some cases, the twins may be separated if the body parts they share are not connected at any major veins or arteries and if they do not share any major organs. Operations are often risky and in many cases both twins do not survive. The decision to have Siamese twins separated is an important one, especially if one twin is more at risk than the other. Sending twins into surgery knowing one may not make it is difficult, although sometimes necessary to save the stronger twin and ensure his or her long-term survival.
The term “Siamese twins” comes from Eng and Chang Bunker, conjoined twins from Thailand, then called Siam. They achieved international fame as teenagers appearing in circus shows, and eventually became successful businessmen in the United States. The term Siamese twins was used to refer to them because they came from Siam. Although still widely used, the term Siamese twins is not considered appropriate. Conjoined twins is the proper name for the condition.
Although researchers don’t know why, conjoined twins are most often male. Even so, most conjoined twins living today are female because girls tend to fare better outside the womb than their male counterparts. The reason for this is not known.