I agree with LeeAnn and this is from two vets
Mammary cancer: Estrogen is one of the primary causes of canine mammary cancer, the most common malignant tumor in dogs. Animals that are spayed prior to one year of age very rarely develop this malignancy. Spaying a dog before her first heat is the best way to significantly reduce the chance your dog will develop mammary cancer. 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=926