From personal experience I would recommend buying a nursing bra before the baby is born. I have found (again from experience) that breasts will not change dramatically in size when the milk comes in. There is the initial engorgement (always a party), but that will subside in 2-5 days and your breasts and their milk will regulate. I would not worry so much about the cup size. If it fits at the end of pregnancy (for me my breasts already much bigger than normal) it will fit for nursing. There may be a little extra cleavage in the beginning, which looks very sexy in the pajamas I will wear for the first three weeks after delivering my child. I would be more concerned about the band size (the circumference of ribcage.) I would allow room for this to decrease in size post birth. Don’t buy a bra that is hooked on its largest setting at the end of pregnancy. You will get smaller.
Under wire or no under wire is the question? Nursing bras are certainly not pretty. Functional is a much better way to describe their aesthetic. With front flaps designed for easy one-handed hooking, and fabric and cut designed for comfort, a nursing bra does its job, but flatter the shape it does not. A little under wire will do wonders to improve the shape of an otherwise shapeless front-snapping cotton bra. However, I am prone to clogged ducts and in the beginning the under wire bra aggravates my problem. I leave the answer to this very pressing question, under wire or no under wire, to you. I personally wear no wire around the house, but if a little vanity and self-esteem are needed I will opt for the wire.
So, buy your bra in your last trimester and don’t be deterred by comfort. There is a reason the bras are designed the way that they are. In the beginning, you are snapping and unsnapping all day and night and that is what these bras do best. Best of luck!