Kells, your breasts can increase when your milk comes in, so just get measured up now and buy a non-underwired teen trainer bra. I'm tiny too, now a 34B, and the training bras are dead comfy! I never had one as a teen (never needed one) and now I either wear these or go without.

Get just two and wash them a lot - maternity bras are expensive and it's too much money to spend if you're going to grow again. I'm still growing and adjusting my new bras, so may well have to get larger maternity ones; I'd hate to spend £50 on a bra and then it not fit for breastfeeding!