This happened to me--I was the "surprise" baby that prompted my parents' marriage. I remember figuring it out and being tormented about it when my parents had problems and finally got a divorce, though I was older than your daughter when that happened. I clearly remember turning 18 and noting to myself that I succeeded in not copying my mom's example of having a baby already.
The first thing that helped me was being able to sit down with my dad and tell him why I was feeling bad about it, that maybe I was the cause of all their problems. It was so comforting to hear my dad say that they would have gotten married even if I wasn't on the way, and that their problems were not because of us kids--it was their own difficulty in getting along with each other.
Another thing that helped was growing up enough to see my parents as ordinary people with all their flaws. And finally, finding faith in God showed me that He had a plan for my life, despite my parents' failures at doing things the best way.
Until she grows up more, you may not be able to prevent her from thinking her simple existence had a part in her parents' problems. But you can keep showing her love and keep telling her how blessed you are that she did arrive to brighten your lives.
When you think she's mature enough, you can talk about how her parents' teenage behavior caused so much difficulty and how she should learn from them and not repeat it. (It's a great real-life abstinence lesson, in my opinion.) But even poor choices can be redeemed. The world was blessed with her presence, and she can do something wonderful with her life.