M.J.
I know this can be a hard one! My 3 yr does it every now and then, it is only a stage. My daughter will do it if she looks at her baby pics, sees a baby talking a lot or something similar. It never lasts long. I give her a minute to get it out and then tell her I enjoyed her as a baby but I also enjoy her as the big girl she is. She stops after that. Normal behavior. Good luck:)