Menstruation signifies the completion of the reproductive cycle in humans. The reverse is the case for canines. When your dog starts bleeding, it means that her reproductive period is about to start.
If you pay attention, you can predict your dog's upcoming season before she begins bleeding. One early symptom is swelling of the vulva, along with increased neediness and clinginess.
The timing of a dog's first heat varies. Some females go into heat at six months, while others might not have their first cycle until a year or later. Even within a single breed, each dog family can have heat cycles that are timed differently.
Heat cycles typically last two to four weeks. Proestrus, which is the first phase of a heat cycle, lasts between seven and ten. A female is not fertile during this phase because she hasn't ovulated yet. As a result, she will ward off any advances. The next phase is called estrus. It is where the female ovulates and is receptive to mating. Estrus can last from five to ten days.
The two remaining phases of a heat cycle are diestrus and anestrus. Diestrus comes immediately after estrus and marks the point at which the female is no longer receptive. Anestrus is the period during which the body prepares for the subsequent pregnancy. During these stages, a dog cannot become pregnant.
It's always a good idea to consult your vet and get your dog examined to proceed with breeding.