Lol it's SUCH a PITA, isn't it? Rest assured, if you didn't already have #2 on board it certainly wouldn't be long before everyone was hitting you up about 'when are you going to give DD a sibling?'.
I was still pregnant with DD2 when everyone started harrassing me about when I planned to go in for #3... I got so tired of repeating that DH and I only ever wanted two kids and just because we were expecting another girl didn't mean we were going to keep popping them out until we got a boy! Two is more than enough for us!

But yeah... sorry, no real advice on how to deal with the well-meaning but frustrating comments like this. I'm sure people don't mean any harm but honestly, do they not think you haven't already heard these comments from 10 other people and that you're probably sick of having to explain your reproductive choices to all and sundry? Congrats on the new bub, hope you have a happy and healthy pregnancy!