Unfortunately — or maybe in this case, fortunately – there is no information available that suggests "Ma" is based on a true story. Director Tate Taylor nor any of the film's stars have made comments about the film's plot taking inspiration from a real-life crime. This Syfy article comments that screenwriter Scotty Landes may have taken inspiration from the famous story of Gypsy Rose Blanchard, but there is no further information to confirm this speculation. Landes did appear on the iHeart radio podcast "Stop! Horror Time!" where the case of Blanchard was discussed, but no insinuation is made that it directly influenced "Ma."
However, in the aforementioned Los Angeles Times article, Taylor did discuss some of the background and setting of "Ma." The film is set in Ohio, but the article makes mention that it was filmed in the director's home state of Mississippi. Taylor explains this is because he wanted the story to feel that it could be anywhere in America saying, "I didn't want it to be specifically Deep South, because then there's this whole other trope-cliché-expectation ... I wanted it to be Anywhere, America. There's a regional neutrality to it." This element adds to the true-crime element of "Ma," it might not be a true story, but it's certainly terrifying to think it could be one.
