I breastfed my son for most of his first year (he's now sixteen months old). Finding a maternity bra that fit well and felt comfortable was not easy. The main difficulty I encountered was that I produced a ton of milk, more than the baby needed, and it caused engorgement early on. I hadn't intended on using an electric breast pump, but I was in so much pain that first week that I made my husband go out and buy one so I could get some relief.
Another consequence of this mega-milkfest (other than feeling like a swollen cow... sad moo) was that my breasts increased significantly in size -- I was probably up to a D cup at my biggest, and normally I am an A or small B cup. I could hardly believe how big I was, and I never actually bought any D-cup bras because it seemed totally impossible that I would stay that size for long enough to use them.
I ended up in a Bravado Body Silk nursing bra in size Small, which is equivalent to a 34B-C. This is an excellent, well-made, very comfortable bra, but the fit seemed to vary depending on the time of day, how much milk my son was taking, and even how much water I drank. At times it felt too tight in the cups, and I could probably have used a Medium.
BabyCentre's guide recommends buying nursing bras when you are in the 36th or 37th week of pregnancy, but for me the correct size was at least two cups bigger once I'd had the baby and started to breastfeed than it was at 37 weeks pregnant. That was probably partly due to my being small-busted to begin with, and partly because I had so much milk.
I had no idea that any of this would happen when I decided to breastfeed. I don't regret the decision, but I learned that when you are on your first child and can't predict how your body will respond to breastfeeding, it's best to leave the tags on anything you buy while you are still pregnant or before your milk comes in, because you may end up a cup size (or even two) larger than you anticipated. You should be able to try on your new nursing bra once you are actually breastfeeding to make sure it fits properly.
Also keep in mind that your breast size may shift, so you want cups that fit even when you are at your fullest; this means choosing a larger cup, because it's better to have a little extra room at times than to be stuck in a too-tight bra. For my next baby, I'll happily wear the Bravado Body Silk bra again, but this time I'll get the Medium just to be safe.