With my first child, I started outgrowing my clothes after a month. Guess I was just too skinny. No one noticed I was pregnant until about month five, but that was likely because I wore loose athletic clothes and t-shirts until the end of the semester - didn't want to think about pregnancy or childbirth until my research paper was over with, LOL.
With later pregnancies, your body knows what to do so it gets into gear quicker; plus you're already stretched out so your body expands.... When I was buying maternity clothes this time around, the sales clerk at the consignment store reassured me that it's completely normal to show more and sooner with later pregnancies than with the first. Expect to go up a couple sizes at least before you're done. First time around, all the stores said that you just buy your prepregnancy size... Yeah, right! I was a small when I started out, and ended up a large. This time around I'd gotten down to a medium (9 months post-partum) and now I'm up to X-large and hoping I don't have to buy anything new in the next three weeks!