Every woman is different. Some women will put on one cup size with pregnancy, but not when nursing. For others, it's the opposite. Some may not gain any size at all and others still may put on a cup size with pregnancy and another when their milk comes in. There is no way to know for sure what size bra you will need. Buy a few pregnancy or maternity nursing bras with a little room for growth, and then buy the rest of the nursing bras you need once the baby is born and you know for sure what size your breasts need. You could also just buy a whole assortment and keep the receipt and return the ones that don't fit.