The bra fitting lady gave me some good adivce and I've only bought one set of bras in the 12 or so months I've been wearing them.

Across the back you should have enough hooks to expand one for every month you have left to go on your preg and one for afterwards. So at month 5 you should have 5 hooks you are able to expand.

In regards to cup size get one that fits but you can comfortable fit your fist in the top of as well as your boob.