I got away with hiding my 1st pregnancy until 26weeks but I was hiding it. I was definately showing but wore clothes that didn't show it. I think I could notice it myself around 18weeks a small but hard round bump.
The 2nd pregnancy it was noticeable from 13-14weeks. I had a pop belly before then but when it was noticeable to others was around that 14week stage.
I am also 6foot tall, trim with a very retroverted uterus and a long torso. So that could contribute to it as much as the backward uterus.