I think the general recommendation is that you should stop wearing underwires by the time you reach 20 weeks because your milk glands are forming/changing and the underwires can cause problems.

I think I got my maternity bras at around 13 or 14 weeks. I couldn't believe it, I had gone from a C cup to an E cup! I'm still wearing the ones I bought then, so for me most of my growing was done early on. I must admit though, I much prefer wearing a sports bra to a maternity bra. I think sports bra helps to minimise your bust, which for me makes me feel better about my appearance. But when bubs come I'm guess I'm going to have to settle for the maternity bras...