Sunday 24 September 2000 Siamese twins are a tragic but rare phenomenon, and few operations to separate them are a success despite big improvements in surgical and imaging techniques. Conjoined twins have been a source of fascination and fear throughout the ages, fuelling horrified speculation among the superstitious as to whether they could be the work of God or the Devil. They occur in about one in every 200,000 live births, after an egg starts to divide into identical twins within the first fortnight of fertilisation but fails to separate completely. That causes the development of twin embryos that are physically joined and often share some organs.
Siamese twins are always identical and of the same sex, as they come from a single egg.
In around half the cases, Siamese twins fail to survive the shock of birth, and around a third survive just one day. Surgeons then have to determine whether they can, or should, intervene to separate the survivors. The best chances are symmetrically conjoined twins, in which there are infants of about equal size who have no birth anomalies except at the area of fusion. If each twin has enough tissue and organs for independent survival, surgery can sometimes separate them successfully. The nightmare scenario is non-symmetrically conjoined twins, as is the case of babies Jodie and Mary.