My opinion:
When she was having sex she was saying that she was ready to be a parent with that dude. Even if she wasn't planning on getting pregnent, that's what she was saying. If she is pregnant, then she should live with the consequence, and have and keep the baby.
Abortion to me is murder, and I don't think adoption is great either for teen mothers. Doesn't anybody watch Cold Case? The post-abortion/adoption depression can kill you because you're so depressed. As the baby grows, most (I'm not saying all), mothers grow to love their child. It's just motherly instinct. I know there are mothers who had sex at 13, and never loved their child/abused them, but I think she should keep the baby. I think the baby will grow on her (no pun intended).
The girl may be better off if she just leaves that house, even if she isn't pregnant. Does she have a grandparent that she could stay with? What about a close friend / other family member? With a step dad who would beat her if she were pregnant, then he could abuse her if she wasn't pregnant, too.