ARSPCA : 10 Reasons for Neutering : Male & Female :

YOUR FEMALE DOG OR CAT WILL LIVE A LONGER, HEALTHIER LIFE.
Spaying—the removal of the ovaries and uterus—is a veterinary procedure performed under general anesthesia that usually requires minimal hospitalization. Spaying a female cat or dog helps prevent pyometra (pus-filled uterus) and breast cancer. Treatment of pyometra requires hospitalization, intravenous fluids and antibiotics. Breast cancer can be fatal in about 50 percent of female dogs and in 90 percent of female cats. Spaying your pet before her first heat offers the best protection from these diseases.