Pretty much as soon as you need to buy a new bra, its best to buy one without the underwire. I started wearing at about ten weeks, just soft cup bras but not the feeding ones yet, cos id look silly!!! I have just recently bought new bras and the lady fitting them thinks they should last til labour and through feeding so have got proper maternity bras.
I have bought three hot milk brand bras, they are a bit different- not the usual unattactive mat bras!! They are sexy and cute and comfy.
Millsy, i think you should prob get some bras without the underwire soon, as i think the wire can affect your milk ducts from early on in pregnancy.
Good luck.
Mel