I am 36w and my boobs haven't changed in size, I'm fairly big busted anyway so have stayed in my underwired bras throughout. I checked with mw and she said it's best to be in a well-fitted supportive bra than to abandon underwired. There are no wires in my bras where the milk ducts are (at the side of cup bit).
Today I went to be fitted for a nursing bra and bought one, I have to say I'm dreading wearing it, I'm a 36H and it's so unsupportive I nearly cried when I tried it on, but I know it's a necessity, I will say that it will be staying in my drawer until needed!