I'd get a nursing tank top if you can, they sell them at Target and at Motherhood Maternity. You don't have any way of knowing what size bra you'll need after the baby arrives.
For example, I was a 36C when I got pregnant with my sons, a 36D during pregnancy, a 38E when they were born and for a few months after, and then went down to a 34DD for the duration of the time they nursed. This time I was a 34D when I got pregnant and have no idea what I am now, I'm sticking to tank tops and jog bra's. But I refuse to buy a nursing bra until a few weeks after the baby is born and I can have some sort of idea what size I'll need that will last longer than a few weeks.