Your breast could increase 1-2 cup sizes in the week or so after delivery. I would wait.
I bought mine at 30 weeks. They recommend you to get one size more of your actual size. I did it with my 1st and Im doing the same this time.
Its a greeat suggestion what ashiepooh sais, try to keep the tags just to be on the safe sad and wifh to exchange them later...
You can go ahead and buy one, i'd keep the tags on it, just incase it's not the right size by the time your milk comes in. Sometimes you boobs will triple insize with milk coming in! Mine sure did! You could always wait until the baby is born too! Maybe buy a few of the nursin tanktops for now, and once your milk is in then go get a nursing bra!