It's a good idea to get fitted or measure yourself) when you're around 36 or 37 weeks pregnant, and buy a nursing bra based on that size. You should buy bras with plenty of room to grow, because your breasts may change dramatically about three days after birth, which is when your milk "comes in."
Breast size fluctuations will settle down after a few weeks of nursing, and your rib band size will begin to return to pre-pregnancy measurements. You might stay at your pre-pregnancy rib band size but at a larger cup size until you finish breastfeeding, although as you've often heard by now, every woman is different, and every pregnancy (and postpartum experience!) is different.