Craniopagus parasiticus is a medical condition in which a baby is born with a parasitic twin head. The extra head doesn't have a functioning brain, which is what differentiates this condition from that of conjoined twins. In effect, the baby is born with the head of its dead twin attached to its body. There have only ever been eight documented cases, and, of these, only three have survived birth. One of these was Rebeca Martinez, born in the Dominican Republic in December 2003, the first baby to undergo an operation to remove the second head. She died on Feb. 7, 2004, after an 11-hour operation.