That is an awful situation - how sad for the poor girl. It's great that she respects and trusts you enough to talk with you, but honestly, as a teacher, I'd talk with your administration and the school counselor to see how comfortable they are with what extent you're getting involved in the situation. It sucks that you can't just be a human who helps this girl, but teaching has taken on a different turn (at least where I teach), and I wouldn't be willing to put my job on the line for it, even though I know I should help.
I like PPs suggestion of providing her resources for pregnancy help centers, WIC, and other available places. I'd also hook her up with the counselor so they can help out too.