Here is one for ya.
Daughter 16 Boyfriend 18.
Dating for a year.
Have to assume they did.
What do you do?

If she is, ask her why she is. Why is she doing it. Listen very carefully and what ever the answer is focus all your energy on making the reason that she is, clear to her. Repeat what she says out loud so she hears you have heard her clearly. So many routes to go depending on her answer but use God, wisdom and patience to reach her and to reason with her. Last resort is birth control because a young woman pregnant having to face THAT decision of an abortion, stays with them for life and can become a definition of self that they can't escape. Key is to work on the 'why' she is doing it. Talking to him as well in the same manner is good too. Honest and open and share your experiences and others onto why they should try to abstain or wait or at the very least not get pregnant.

Do not get mad that will shut her down. Her answer will tell you where she is coming from. 'everyone's doing it' 'it's not a big deal' 'I love him' "he wants it' " i'm not a tease'....these are not the right reasons but be gentle. Ask a professional for advice if you need it but don't close the communication pathways by getting angry. That will set in motion her not listening but rebelling.