There are four stages that make up the reproductive cycle of female dogs. These are:
Proestrus Phase: This is the start of the heat cycle in female dogs. It usually occurs over a period of 7 to 10 days. During this phase, the vulva swells and blood flows from it. Although males will be attracted to her, the female will not let them mount her.
Estrus Phase: This is known as the mating or ‘heat’ period, and lasts for about 5 to 10 days. The female dog will attract and allow males to mount her. After 2 to 3 days after mating, ovulation takes place.
Diestrus Phase: This is the pregnancy period, which spans 10 to 140 days after the dog was on heat.
Anestrus: This is the period between the diestrus phase and the proceeding heat cycle.
There are usually two heat cycles per year for female dogs that haven’t been spayed. These heat periods are six months apart, and last two to three weeks each. It is during these phases that the female dog can get pregnant. There are cases where the heat periods last much longer. The initial heat period happens when the female dog is between 6 to 15 months old, although this is dependent on the dog size. Larger breeds tend to reach their heat periods much later in life.
The male of the species does not have a sexual phase like the female. They just react to the females who are in heat, no matter the time of year. Male dogs are mainly fertile when fully grown.