Dog Reproductive Cycle

The female dog usually reaches sexual maturity at nine months of age, although this can vary from as early as six months to as late as twelve. At this time she begins her heat cycle (also referred to as her "season" or the estrus period), which lasts approximately 21 days.

There are various stages in the heat cycle. Prior to its onset, the female is restless and exhibits an increased appetite. This preheat period generally lasts about 5 days, during which time the vulva begins to swell, often to several times its normal size. This period is followed by the onset of heat, which is characterized by a bloody discharge that lasts for several days and then begins to diminish. Approximately 9 days after the first signs of discharge, the dog becomes playful with other dogs. However, she will accept a male for mating a few days later, around days eleven through 18 of the heat cycle. At this time she will become quite aggressive and seek out a male for mating. As the cycle proceeds (days 19 through 21) she will still be attractive to males but will no longer permit a mating. This mating cycle can vary quite markedly form dog to dog and breed to breed, but it is still characterized by the various phases. The heat cycle occurs approximately every 6 months, although this can vary among females. The male dog, commonly called the "stud," usually reaches sexual maturity at 6 to 8 months of age. Unlike the female, the male dog is able to breed at any time of the year. He does not experience the so-called heat cycles.