I can't tell you what to do since I have not been in your situation. But if I was in the girl's shoes I'd rather have you tell me straight out before things get too serious. I would feel that if you can keep something that big and personal a secret, what else would you be keeping from me? It would make me doubt everything about you and about our relationship and that's not good for any relationship, be it friendship or romantic.

Plus, it's not fair to take the choice away from her. In order to make an informed decision she needs all the facts up front. There is nothing stopping her from braking things off with you even if y'all have been dating for a long time. If this is a "no-no" for her it's not going to matter if you've been dating for 2 days or 2 years.

Just wanted to give my two cents, take it or leave it. But I do wish you the best of luck.