Proestrus—Start of heat. Lasts 7 to 10 days. Vulva swells and blood flows. Females attract males but will not allow mounting.

Estrus—Mating period. Lasts 5 to 10 days. Blood flow lessens and then stops. Females attract and accept males. Ovulation occurs during this time, usually 2 to 3 days after mating.

Diestrus. The period 10 to 140 days after heat, when the dog is either pregnant or in a resting phase.

Anestrus. The resting period between diestrus and the next heat cycle.