I was worried about this too. Terrified, actually. Fortunately I am almost done and my feet have not grown. Right now they're kind of swollen and the only shoes that I'm actually comfortable in are my Pumas, but that's pretty normal. I didn't have any problems at all until about the 32nd or 33rd week. I know it's different for everyone though; one of my co-workers gained a size with each one of her kids!
With my first pregnancy I went up 1/2 a size, but I lost a ton of weight a year after she was born and went back down. With this pregnancy my shoe size did not change at all.
Not to scare you but...I was an 8 before I had my first son then I became a 81/2 after. I then had my 2nd son and went from an 81/2 to a 9. (Now completely unrelated after having foot surgery I became a 91/2.)
Now, when I had my first son I was only 17 so I guess it's possible that my body/foot was still doing it's normal growth, but the second time, well there is no denying that.
my feet swelled a bit, enough to make it so i couldn't wear most of my pre-preg shoes...but now they seem to have gone back down. (though i haven't tried on all of my shoes to make sure they work..)