I don't know what the rules are with starting to show - but I am only just starting to show now at almost 21 weeks (and it's still only enough for myself and DH to notice). I'm not the samle size as you - before I fell pregnant, I was very overweight, and I carry a lot of my extra weight around the tummy - so I'd say that has a lot to do with me only just starting to show.

As far as I understand, when you start to show is dependent on your height, weight, length of torso... and I think chance has a lot to do with it! I knew a girl who was very slim, and she didn't start showing until she was 7-8 months! She didn't really look pregnant.

Good luck with your pregnancy!