You should not be wearing underwire bras from 16wks pregnant because they can cause blocked milk ducts and lead to problems breast feeding and breast infections (or so my midwife told me).
As for maternity bra's, it depends how much money you have, I have got some hotmilk ones and they have been lovely, they are the only comfortable bras I have had to wear throughout this pregnancy. I also have some bendon ones which I wore with my DS and they were good once I had bubs but I cant remember what I wore when preg with him. Depending on how much money you have and where you are from (I am in NZ) then Bendon has bought out a new range called Bendon Motherlove and it has both breastfeeding bras and bras designed for pregnancy. They look lovely and I hear are really comfy. It just depends if you want to buy bras now and then again to breastfeed, I personally couldnt be bothered, I just wanted whatever was comfortable and didnt want to have to buy more later!