This is america, everyone is having sex at such a young age, its just life, and I mean, no matter how many "sex talks" you give them, it won't stop them. It'll give them ideas of the dangers and risks, but it won't neccesarily stop them. My parents never gave me the "sex talk".
I was 13 when I got pregnant. I felt alone and as if my parents would think less of me. Of course they would, I mean, 13 and having sex.
Please give her your support and don't yell at her. It's okay to be angry but if she is pregnant, she will want your support and she will want to know that you're there for her no matter what. Pregnancy at such a young age is hard to deal with, so she'll need your love.