I do think perception has something to do with it though. I can choose to take someone's comment as being positive in intent, and feel good that they've said it, or I can perceive it as rude or negative and have it ruin my day.

Either way, responding to it as if they meant it positively is a win-win - if it's actually meant positively then I have responded appropriately, but if it's meant negatively then it completely takes the wind out of someone's sails to be replied to as if they were giving you a compliment. If someone says in a derogatory way that you've fallen pregnant again quickly, then replying 'I know, isn't it wonderful, we wanted our kids to be close together in age' gives them no ammunition at all.

OK, some people are just out and out rude and obnoxious, in word, tone and manner. But the majority (even if they could use a good thumping with the tact stick) are just trying to make conversation. To be honest, we'd (we being pregnant women in general) be more peeved if they said nothing at all and totally ignored the pregnancy, which is not that unlikely if they feel they are unable to say anything without potentially offending us.