I'm seconding what Kerry said - as soon as you start to get uncomfortable in your regular bras, go to a lingerie shop and get properly fitted, with an experienced attendant. Maternity bras should be fitted a little differently to regular bras, as your boobs expand and deflate depending on your milk flow and baby feeding!

As to when you will need them, well, with my first pregnancy I didn't need a change of bra right up until I had the baby. With this pregnancy, by the time I was three months pregnant I needed to up a cup size, and up another again at seven months! It's a very individual thing.

Kerrie