If I sense someone is interfering (as opposed to just crap at conversation!) I always reply with some variation of: "When do YOU think we should?" and then let them talk, and talk, and talk.
Often they forget about whether or not you've actually answered. When they do wind down you can say something like "Thank you for the tip, that's interesting." and then change the subject (you've usually had time to think of something else by then).
This works for all sorts of questions that you don't want to answer:
"How many children are you going to have?" - "How many do YOU think we should have?"
"When are you going to start a family/have another child?" - "When do YOU recommend we should?"
"Are you hoping for a boy/girl next time?" - What do YOU think would be best?"
"Are you planning to lose weight at some point?" - "When do YOU think I should lose weight?!"
You don't have to be sarcastic with it - it's just a way to allow them to talk about something they're interested in, when you'd rather not. It's also quite a subtle way to save THEIR embarrassment when they've accidentally touched on a really painful subject.
Finally, it helps to flush out the really determined nosey parkers, because they will keep batting the question back at you or even point out that you're being evasive - then you can be more direct if you wish, safe in the knowledge that you are dealing with a beaky sod, rather than a foot-in-mouth person.