It's Alive (1974) was filmed and edited simultaneously with another Larry Cohen film, Hell Up in Harlem (1973), which was shot on the weekends during the production of It's Alive (1974). This means that many of the same cast and crew put in consecutive seven-day work weeks to create both of these films.
Rick Baker made an oversized body cast of the monster baby which he put his wife into. Larry Cohen questioned on how he could put his wife in there, to which Baker responded that he needed someone available 24 hrs. a day, and because Baker worked out of his garage and needed someone to put the cast on, so Baker made her the model for the monster baby.
Larry Cohen called Rick Baker on the telephone while Baker was at the home of Dick Smith where he was working on the special effects for the film The Exorcist (1973). During that initial phone call, Cohen pitched the film's concept to Baker, and suggested building a monster baby suit that could be worn by Cohen's cat, or "a chicken or something [...] maybe two chickens!"
For close-up shots, Rick Baker created a full-head monster baby mask, a pair of gloves and a partial body suit, which were worn by Baker's then-girlfriend and later wife Elaine Alexander.
Larry Cohen originally planned to cast Anthony Perkins and Janet Leigh in the lead roles, thereby creating a mini-Psycho (1960) reunion.