Conjoined Twins (also known as Siamese Twins) are identical twins whose bodies are joined in utero. This rare phenomenon occurs in every 50,00 to 100,000 births.
Depending on the point of attachment and the internal parts that are shared, surgery to separate conjoined twins may range from relatively simple to extremely complex. 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.
Recently the 11-months old Sudan Twin girls Ritag and Rital, were successfully separated at a British hospital. Ritag and Rital were so calles craniopagus twins, meaning they were born joined at the head: