Today I told a mum at DS2's crèche that we are expecting DC3 in the autumn. Her first response was "Oh! Was it planned?". Nobody said this to me when we announced our first two but it has happened a few times now this time round. I wouldn't dream of saying this to anyone, I think it's quite rude. A simple "congratulations" would suffice IMO.
I am 40 by the way and I don't know her at all well, just to say hello to....