Good thread, because I'd been wondering the same thing!

This pg my trousers started feeling a bit too snug at 9 weeks and my usual tops/shirts started sitting tighter across my tummy at about the same time. I started wearing maternity clothes at 11 weeks - felt weird buying winter maternity clothes before the end of first trimester, but sometimes ya gotta do what ya gotta do!

With DS, I didn't even need maternity clothes until approx 16 weeks!