Bypass the discount stores and sale racks and have a fitting done by an experienced fitter at a well stocked lingerie department or at a specialty shop. A well seasoned fitting specialist should be able to tell approximately how much extra room and support will be needed. Being fitted for a bra is a service that is provided at no extra charge so take the offer to be fitted and see how good a bra will feel when it fits properly. A good bra is not cheap. You can just buy a regular bra in a larger size, but you may regret it.
Maternity bras offer wider straps for more support and extra coverage.
Extra rows of fasteners will allow the bra to grow with the pregnancy this is an important feature when you consider that the average womans rib cage expands one to two bra sizes. Dont go for an underwire style bra. They can cause clogged milk ducts if they press too hard on breast tissue. As for the racy styles to enhance cleavage, pass on these also. They will still be around after the baby is born.