Conjoined twins are twins whose bodies are joined together at birth. This happens where the zygote of identical twins fails to completely separate. Conjoined twins occur in an estimated one in 200,000 births, with approximately half being stillborn. The overall survival rate for conjoined twins is between 5% and 25%. Conjoined twins are more likely to be female (70-75%).

The term Siamese Twins comes from what are probably the most famous pair, Chang and Eng Bunker (1811-1874), Chinese Americans born in Siam, now Thailand. The term is frequently used as a synonym for conjoined twins. The earliest known case of conjoined twins dates from the 1100s, the British sisters Mary and Eliza Chulkhurst.

There are sveral different types of conjoined twin:

  • Thoracopagus: bodies fused in the thorax (35-40% of cases)
  • Omphalopagus: joined at the lower chest (34% of cases)
    • Xiphopagous: bodies fused in the xiphoid cartilage, e.g., Chang and Eng
  • Pygopagus (illeopagus): joined, usually back to back, to the buttocks (19% of conjoined twins)
  • Cephalopagus: heads fused, bodies separated
    • Cephalothoracopagus: bodies fused in the head and thorax
  • Craniopagus: skulls fused (2%)

In some cases, parts of the brain have been known to be shared between conjoined twins joined at the head.

Occasionally one of the twins will fail to develop properly, effectively acting as a parasite upon the normally developed twin.

Separating conjoined twins

Some pairs, depending on the degree of conjunction - in particular, the degree to which they share internal organs - can be separated by surgery.

In 2003 two women from Iran, Ladan and Laleh Bijani, who were joined at the head but had separate brains (craniopagus) were surgically separated in Singapore, despite surgeons' warnings that the operation could be fatal to one or both. Both women died during surgery on July 8, 2003.

One ethical issue with separation is when the operation will result in the death of one twin (for example, in the case where they are sharing a heart.) A notable case was that of Rina and Michaelangelo Attard, from the Maltese island of Gozo; despite the opposition of the Attards, the High Court of England and Wales ruled that their twin daughters should be separated, even though this would cause the death of Rosie, the weaker twin.

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