Well cupcakes I'm in the same boat and have come up with the following reply, at the recommendation of Dh, as he was quite shocked at disgusting reactions as well:
offending party: OMG how will you cope?
me: I'm sure I'll cope thank you, I'm an excellent mother and have produced 2 well adjusted, happy children, so I'm obviously doing something right aren't I? (cue v.v.v.smug look)
To the gits who ask 'was it planned?' I reply as follows:
git: oh dear, was it planned?
me: was what planned?
git: the baby!
me: Oh yes, of course this pg is planned, were yours?
So far responses have ranged from slightly uncomfortable to be in front of me to fuming at my smugness.
Dh summed it up nicely I thought, saying if anyone tells you they're pg then the only acceptable reply is CONGRATULATIONS! regardless of how well you think you know the person. They will let you know if they are not happy, but always err on the side of "I'm happy for you" when told.
Personally I've always held that if people aren't pleased to be pg they won't be advertising the fact they are until they've figured out what they will do