Let's get this straight - your bra should be like your best friend. If your best friend doesn't support you, pokes you constantly, and is just a general pain to be around, then you probably need a new best friend, and the same goes for your bra. Your friendly, everyday underwired bra should not be hurting you - at all!
The right time for you to be fitted into a maternity bra is when your breasts begin to noticeably swell to the point that your existing bras no longer fit or become uncomfortable. This is usually towards the end of the first trimester and usually about the same time you are busting out of your jeans.