I was going to PM you and say pretty much the same thing p-jet just did. I see this story everyday, really, it is what I do. Adoption is by far the best choice for everyone ESPECIALLY the baby and your daughter. It is so hard for the girl and the family to focus on the future instead of the present, but that is what needs to happen.
Explain that:
Young forced marriages don't last and divorces just set back everything financially and emotionally.
Large majority of prisoners are children of single parent households
75% of runaways are children of single parent households
Most single mothers don't finish college therefore will never make what they otherwise would have.
Much harder to get married if you already have a child.
"Do you love your father?" "Then why would you not choose the best man possible for you own child?" or worse "Why would you choose to raise a baby that you know will not have a father around?"
Wouldn't the baby have more opportunity and probability of having a great life with 2 parents that are already firmly established with good careers and a comfortable house?
Above all don't got crazy on her because you need to be able to talk to her openly and she needs to not fear you. She is plenty scared already I am sure. Preg tests are pretty reliable, especially if used a few weeks into the pregnancy, but get her to a doctor for a check up anyway. If she is, she starts working on that pregnancy as JOB 1. Exercise, diet, reading medical literature, and getting AHEAD IN SCHOOL. She will be missing a lot, she needs to move ahead in her studies now! Your schools will have special programs such as Independent Study, use them.
Get the boy involved financially if he is over 18. He needs to sweat too. He needs to want to sign off on adoption AND he needs to learn not to impregnate anybody else.
I am sorry if I come off as an Adoption Nazi but I was adopted, and many of my circle of friends and co-workers are too. I am on the front lines with this daily, and I see what happens when it goes both ways. Keeping the baby CAN work, but it is rare and difficult for everyone.
She is so young, help her to see that she will still have 20 years or more to do it right again, but in a way that maximizes her chances for success. In the end that is all that everyone, including people like me that have never even met her, wish for her.
Tell her a bunch of cool peeps all over the country are pulling for her to make some painful decisions that will allow a baby to have the best life possible from the very start. I know people her would tell her that the "right" decisions are often the hardest ones to make.