The Nikki Sheridan Series by Shirley Brinkerhoff  is one of the best series I think has ever been written for teens

I haven't read that series, but Robert Liparulo's Dreamhouse Kings series is one of the best I've ever read. Frank Peretti's Cooper Kids series or Veritas Project books are good, too.

What is the Nikki Sheridan series about?

The Nikki Sheridan series started out that she makes the mistake of going all the way with the popular boy at school, only once, and ends up pregnant.  It goes through her finding out and deciding whether to abort the baby, keep it or give it up for adoption.  Her parents try to force her to have an abortion, she stays with her grandparents, deals with the situation and later reconnects with her parents.  I wish more teens would read these and realize that you CAN get pregnant the first time and that pregnancy during your single years is never easy.