Please get her spayed before she has her first heat. By doing so you will practically eliminate her chance of getting mammary cancer later on.
The risk of malignant mammary tumors in dogs spayed prior to their first heat is 0.05%. It is 8% for dog spayed after one heat, and 26% in dogs spayed after their second heat. http://www.peteducation.com/article....&articleid=460