I would suggest telling her as soon as possible. She has a very special relationship with you that will always be important to her. The true story of your becoming a family needs to be a part of her life as she grows. She will understand it and appreciate it in different ways as she grows. I was adopted and have known that fact as long as I can remember. My mom (who raised me) always described things to me in terms of my having a very big family. My being given up for adoption was only conveyed to me in positive ways. "Your birthmother loved you so she gave you to parents who could take the best care of you, etc." How beautiful that your daughter has you for a mother. I'm sure you'll know just how to share the story of your finding each other in a way she will be able to take in and think thoughtfully about throughout her life.