The reproductive cycle in female dogs has 4 phases. Female dogs that have not been spayed (bitches) typically have 2 estrus or “heat” periods per year (about 6 months apart), each lasting about 2 to 3 weeks. In some dogs, the intervals between estrus are much longer. The first heat occurs between 6 and 15 months of age, depending on the size of the dog (later in larger breeds). Females can become pregnant during their first heat or any later heat period.

Estrous (Heat) Cycle in Female Dogs
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.

Male dogs do not have a sexual “cycle.” Rather, they respond to females in heat at any time of year. Males are most fertile when fully mature.

Last full review/revision July 2011 by John A. Bukowski, DVM, MPH, PhD; Susan Aiello