While these are standard bra sizes, make sure you check for guidelines at the specific retailer if you decide to buy your bra online. (easybabylife.com)
MotherWear UK has directions, and they sell a kit for converting standard bras to nursing bras, helpful if you're in the UK. (sleepingbaby.net)
The Dairy Fairy Arden does not use standard bra sizing so to order the perfect fit, be sure to check out their sizing chart and measuring guidelines. (tinyfry.com)