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.
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.