Craniopagus parasiticus

Craniopagus parasiticus is an extremely rare congenital condition in which an undeveloped parasitic twin head, lacking a functional body, remains attached to the head of a fully developed twin. It results from incomplete separation of identical twins early in embryonic development, a process whose exact mechanism is still not fully understood; one twin continues typical growth while the other's development is severely arrested. The condition occurs in an estimated four to six of every ten million births, and fewer than a dozen cases are documented in detail in the medical literature even though roughly eighty cases have been reported overall. Very few affected infants are born alive, and for those who survive delivery the only treatment is surgical removal of the parasitic twin, an operation that has so far produced poor outcomes, with no documented survivor reaching a second birthday.