How to talk about ?

I am 17 and have been going out with my boyfriend (18) for a little over 7 months now. We are both really good kids and are really in love. (this is different than anything else we have ever experienced) about 4 months ago we began to have . I promised my mom that i would tell her before anything like that happened, but i did not. Now I am almost 18 (in less than a month) and i would feel better if i told her...but I am petrified!!!! I dont know how to tell her. My boyfriend said he would be with me if i wanted him to. His parents have not been told but ume and dont say anything to us other than be safe (which we are). So I guess I am asking for help of how to break the news to her because i am scared of the consequences (her reaction and going to a gyno) please help!!!!