It could have been all in her head
The film does leave that possibility open.
shareNo it doesn't. There is nothing in the cinematic language of the film that opens up that possibility.
shareOn the contrary, the film is deliberately ambiguous regarding any supernatural elements. According to Polanski:
"one aspect of Rosemary's Baby bothered me. The book was an outstandingly well-constructed thriller...Being an agnostic, however, I no more believed in Satan as evil incarnate than I believed in a personal god; the whole idea conflicted with my rational view of the world. For credibility's sake, I decided that there would have to be a loophole: the possibility that Rosemary's supernatural experiences were figments of her imagination. The entire story, as seen through her eyes, could have been a chain of only superficially sinister coincidences, a product of her feverish fancies...That is why a thread of deliberate ambiguity runs throughout the film."
In terms of this film, I don't think that's the case at all. But Ira Levin did write a sequel to the novel in the 90s called Son of Rosemary (SPOILER ALERT, stop reading if you plan on reading that novel) and at the end Rosemary wakes up and the events of both Rosemary's Baby and Son of Rosemary are revealed to have been a dream. Talk about the ultimate screw you ending lol!
shareI absolutely felt cheated after reading the sequel! I gave the book away and swore that I would banish it from my mind! LOL!
Seriously, he shouldn't have bothered writing that second book if he was going write such a crappy ending.