sadly I think there might be no polite answer - as the question is nosy/annoying with a hint of criticism and also border line rude, I don't think you owe anyone a polite reply. or any reply indeed!
something that you find funny/shocking might do the trick.
I think the classic "why do you want to know?" or "you'll be the first to know" tend to shut people up, but must be delivered with the most sugary smile!
a quick pick of the countless idiotic questions I've been regularly asked so far having had 5 boys then a girl (my answer/s in brackets, most were genuine answers I actually gave! I know, brave huh?. Enjoy!
- is baby going to be early or late? (baby's not said yet)
- when is he finally going to have teeth? (no teeth, we are going straight for dentures)
- is he going to stay blond? (that depends on his stylist)
- when is he/she going to sit/crawl/sleep through the night? (let me check the schedule...yep,that will be next month, but right now we are going to a lecture about the double-helix structure of DNA and s/he is the keynote speaker)
- is it a boy or a girl (yes, one of those, it would be awful to give birth to crocodile)
- is she going to be a tomboy? (she's not giving interviews)
- isn't your bump/belly massive? (oh, I didn't notice, thanks for pointing it out, here, use my loudspeaker so everyone can hear you)
- I didn't know you were pregnant/had another one! (sorry, my fault - what's your name again??)
- are you stopping now or you are having more? (you are the first person to ask me that question, how odd!)
And The Award Goes To.... my personal favourites when around 37 weeks pregnant:
- still no baby? (what gave it away?/ oh, sorry, does that upset you or you schedule? I'm actually due next month, but to ease your discomfort of having to wait I'll see if I can hurry up)
- are you still here? (no, it's a hologram/ yes, where would I be? I'm standing right here, speaking to you as we speak....)
hope this is helpful and at least made you smile.
feel free to try them out - you can amuse yourself by writing the reactions down!