I think it depends on your body type, your uterus pointing towards the back or front and if you have had any previous pregnancies. I watched a few episodes of "i didnt know i was pregnant" which is purely trashy american tv, but some of the women on that show never got a bump! Although it looked like a lot of the women were overweight and the one skinny girl i saw delivered at estimated 30 weeks. They showed a picture of her in a bikini taken 2 months before she gave birth and there was no bump.
I'm 12 weeks with my first and have a chubbier tummy than I have ever had, although I've got hg so have lost more than a stone in my first trimester. Not really a "baby bump" yet. My body shape has changed so much... legs are getting so skinny, my bum is getting so flat and tummy getting tubby, boobs are getting massive. Haha