'Sequel' to Rosemary's Baby
I know I'm really going out on a limb with this, but I like to think of this film as a sequel to Rosemary's Baby. As Mary, Mia is still sporting the same hairdo she had as Rosemary and even wears clothes that are similar to the ones she wore as Rosemary (e.g. that brown dress with the lace collar). I imagine that Rosemary managed to gain her freedom from Guy and the coven, shortened her name, and decided to try to rebuild her life.
Of course, I know that this isn't really a sequel to RB. This is just a fun bit of speculation that I like to indulge in.