Unspayed female dogs usually go through two heat periods each year. During her heat period, your female dog may drip blood. She will also make every effort to sneak out to find a mate. As a result, she is at high risk for being hit by a car.
Unspayed female dogs suffer from a high incidence of mammary tumors, false pregnancies, uterine infections, and reproductive cancers. Breast tumors are the most common type of cancer in dogs. One out of every four unspayed dogs will get breast cancer, and half of the tumors are malignant. Unspayed dogs are also prone to pyometra, a life-threatening infection of the uterus. Spaying removes the possibility of diseases of the ovaries and uterus, and comes close to eliminating the chance of mammary tumors.
If your dog was in heat when she was spayed, she will continue to attract males during this time. Keep her away from male dogs during her recovery so that she isn't accidentally injured.