Although female dogs start going into heat at the age of 6 months, they should not be bred until their second season, which is usually six months later. Males reach sexual maturity sometime between the ages of 12 to 15 months and can breed anytime. However, small breeds usually reach maturity earlier than large breeds.
Before breeding two dogs together, test them both for brucellosis, a bacterial infection that can cause sterility or miscarriages. A female dog being prepared for breeding should be at a healthy weight with muscle tone and ideally have a temperament suited for nurturing puppies. She should be up-to-date on her vaccinations and have been tested and treated for parasites.
The breeding cycle is split into four periods. It begins with proestrus, which lasts about nine days. At this stage, a female dog's vulva starts to swell, and she begins to attract male dogs, but she does not allow breeding. She may also have bloody vaginal discharge. Proestrus is followed by estrus, lasting another nine days. In this stage, the female is fertile and accepts males. The following two to three months are known as diestrus, when the hormone progesterone takes control of the reproductive tract. This occurs whether or not the dog is pregnant. Finally, anestrus occurs over the next three to four months, during which no sexual activity occurs.