The best time to spay a female dog in is the first four months of her life. Spaying will also prevent cancer and many other problems in future. If you do not want your female dog to have puppies, then it must be fixed before she has them. The age for fixing females can be discussed with your veterinarian or animal hospital staff member who performed the surgery on your pet. At any age, if there are no medical issues involved, an elective surgery should be performed by a veterinarian which would include anesthesia and pain control for all animals undergoing surgery..