I found that I sort of lost control of my tummy muscles which I guess is what you are saying that you "can't hold it in anymore"... I'm average weight for my hieght, but I had a definate bump from my tummy muscles being slack at 9-10 weeks and was in maternity pants at 11 weeks.

BTW - where in Scotland are you? I love it over there, was in Edinburgh a few years back and it felt like I was "home"!