My SIL had a surprise pregnancy - found out about the 4th baby when her 3rd baby was only a few months old - they are 13 months apart (1st 2 babies were twins and a few years older). So talk about comments...after initially having twins, they waited a few years, then got preg with #3...they were planning to be done, but #4 took them completely by surprise. SIL was nursing and on the mini-pill.
Anyway she also was so irritated by peoples' comments during her 4th pregnancy...she felt like they were non-stop. She usually just smiled and didn't say anything back...but toward the end of that 4th pregnancy she started telling people she was due with a baby elephant - they have a 2 year gestation. That was actually pretty hilarious and helped to stop the comments.
Don't worry about what people think - I really don't know why there are so many idiots out there, but there are. At work I'd probably say something like 'stay away from the water here,' or 'after this the factory is closed,' or my personal favorite, 'I'm just excited about getting another 3 months off.'
Congrats on the new addition. :)