How to answer the annoying question everyone always asks.

How do I politely and without telling me whole life story respond to "when are you going to have another baby?" If it were as easy as they make it sound I would have at least three children now. My MIL, who I finally told my whole medical history to, is the biggest offender. My SO has even told her to stop asking because of how much it upsets me, but she won't stop. I have random strangers off the street ask the same thing. Why do people think it is their business and how should I handle these obtrusive inquiries?