Why not find a few books and articles on natural birth and why you chose that route and give them to her to read. Say "I know you are concerned but I want you to be informed on why I made this choice for my body and my baby." What she does with that is then up to her.
I want to add that as a parent you will always be doing something wrong or that others (mainly grandparents) don't agree with so it's best to discuss with your husband now how the two of you will handle these situations now and in the future. Yes it would be nice if people would mind their own business but IME they don't. Your MIL is already butting in, I can't imagine it will change unless you and DH are on the same page about it and how to handle it.
High Priestess JessMess, follower of the Goddess of the Coiling Way and Confiscator of Concoctions in the Order of the Curly Crusaders