Described by Guillermo del Toro as being a sibling film to O Labirinto do Fauno (2006) (this being the masculine "brother" film, and Pan's as the feminine "sister" film).
Guillermo del Toro has said of the characters in this film: "All the good guys in the film had the initials 'C' and I wanted to have Jacinto and Jaime to share the same initial because Jaime is a bully that chooses to change--unlike his grown up counterpart."
The design of the ghost was inspired by the white-faced spirits of Japanese horror films like Ju-on (2000) and Ring - O Chamado (1998).
Was strongly inspired by the director's personal memories, especially his relationship with his uncle, who supposedly came back as a ghost.
The film's title refers to the medical condition of spina bifida.