You can buy the same size nursing bra that you would in your regular or pre-pregnancy bras. However, because your breasts can grow by a cup size or more because of your milk supply, it's usually a good idea to also buy a bra that's one to two cup sizes larger than your regular bra size. if you're unsure, consider opting for bras made from stretch material that's more forgiving and can accommodate a fluctuating bust line.