I'd say about 11 weeks was it for me, and by about 15 I was obviously obviously pregnant. That's when someone first offered me a seat on public transport. I have no idea why I showed so early. Fwiw, my belly got badly stretchmarked as it just popped and popped. I also got to the point at about 37 weeks where my measurements stayed the same. My DD was very squashed when she was born, her head plates were overlapped, her face was literally lopsided and she has hip dysplasia on the squashed side. I think my body carried too long, her placenta was grainy and she looked overcooked. Your uterus position is going to make a huge difference to how you show too, some women just carry their babies right inside of them.
ETA I can't link to my preg album from my phone, but if any of my FB friends want to, go ahead :) I also wanted to add that I'm quite short, but a little overweight.