In the last few weeks, I've noticed a resurgence in those yellow-and-black "baby on board" signs in car windows. I gnash my teeth every time I see one.
I think it's of the egocentrism that these notices display: We're important, because we had a baby! Lots of people have babies. The question is not how other people react to that, but how well you do as a parent. Sticking a plastic sign in the rear window of your car doesn't mean much compared to how you treat your child--and if you think it does, then maybe you don't deserve to have one.
Then, too, it might be that what bothers me is the suspicion that those people are already living their lives through the children.
(It's especially annoying, of course, when one of the baby-on-board folks drives like an idiot.)