I certainly wouldnt just go up to her and say " I heard you talking that you think your pregnant" SHe will think you were trying to listen, or invading her privacy....
Maybe you could try just bringing it up in a way, to tell her that you want her to know she can tell you ANYTHING, without you being angry or upset at her.....just make that known to her, maybe she will come to you , if in fact she is pregnant.
I mean, you definately need to get yourself involved, thats always important,esp in a teens life, but you have to be very careful.....Obviously she looks up to you enough, and stuff, cuz she brings her friends to your house to hang out.
Just play it cool for a while and see what happens....maybe make her mother aware of what you heard? Of course, if her mom will flip out on here, thats not good either.
Just use your own judegment.