Well... i'd be concerned, because i've never had sex before... and then I'd just tell them, probably. Explain everything. They probably wouldn't believe me... but...
I'd be depressed, knowing I'm a preganant 13 year old girl and a Christian. I would avoid talking to my parents because I'd be ashamed. :/ (Flying Mint Bunny)
Calmly tell them i'm pregnant, and i don't know who the father is.
after that i'd go in my room and freak out, noting i'm 13 and very had any physical contact with a boy besides my brothers and dad, and that wasn't anything close to sex.