What do I need to know when buying maternity bras?

Buy expanders. The best advice i got was to buy these little expanders for the back of the bras. Get a saleswoman who has had kids to help you.
Comfort. Buy a bra that fits you well, and you are comfortable with. They will have a flap to make your nipple readily accessible for breast feeding. Good luck.
Professional fitting. Would encourage you to go in person to a specialty bra store to purchase maternity bras. Up to 85% of american women are wearing incorrect bra size. The professionals at a specialty bra stored can ensure that you have the correct band and cup size as well the best design to fit your need. You will also be taught how to properly settle your breasts into the bra. It is worth the effort ; will ^ your.