"They" say to get fitted for a maternity bra at or around 4 months or 16 weeks. I guess that really does depend on how much you have grown. I went and got fitted at Myres, somewhere else you can go to is bras and things or another speciality store. You don' have to buy their bras just tell them you want to get fitted so you know what size you need.
I'm bad, most days I don't wear mine, there is so much extra material in them and they make me all hot and sweaty. I'm normally on in a 12A but atm need a 14D so the bras are huge compared to what I'm used too.

Take care
Trish