I only started buying in the last few weeks. And even so, I had to change my size as it went up 2 cup sizes 2 wks aft birth when my milk supply increased (not a gd thing for an already big chested mama). So best to buy just 2 or 3 nearer to EDD just in case ur size goes up again once milk comes in.
I started using a maternity bra in my first trimester as using a wired bra was getting uncomfortable for me.