Martínez was a parasitic twin, an extremely rare case where one of two conjoined twins stop growing as a fetus. She had a condition known as craniopagus parasiticus. There have only been eight known cases of this condition through history, and Rebeca was the first of those eight not to die before birth.

Rebeca Martínez went into surgery at Santo Domingo, on February 6, to try to remove her extra head. Because she had been the first person to be born with her condition, this was also the first operation of its type in history. Doctor Jorge Lazareff led the historic operation, which lasted 11 hours. On the morning of February 7, she passed away from complications related to the surgery.