Yes tell her before someone else does. I had confided in my cousin and we got into a fight her being spiteful she told my mom it was horrible.
I agree just say it! That's what I.did and I felt so much better.
Just tell her flat out, probably when it's just you and her. I was 17 when I got pregnant, married, and still scared, she will accept you, and your baby no matter what, shes your mom, and you'll get SUCH a better understanding of just how much your mom loves you when you have your baby, my mom has loved my son just like i have since the day he was born, it will all work out! good luck hun!