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.
Should you decide to breed your female, you should not attempt this until at least her second or third heat. The female is still growing at the time of the first heat, and she is not physically prepared for the rigors of raising a litter. To prevent an unspayed female from mating during her heat period, you will need to take some precautions. Never leave her unattended outside, even if tied, as males will go to great lengths to get to her. This is especially important in the middle of her cycle when she will accept and encourage a mating. To prevent the local males from discovering that she is in heat, take her away from her immediate surroundings to relive herself, as the urine of a female dog in heat gives off a scent that is particularly attractive to male dogs. It is best to take her out in the car to a park or public spot, being very careful, always, to contain her on a leash and clean up all solid wastes. This may sound like a lot of trouble, but it may keep the male from congregating around your house in the hopes of getting to your female.
Many people feel it easier to board the female in heat at a local kennel until the season is over, but this is by far the most expensive alternative. If you should choose such an option, be sure to make it clear that the female is in heat and must be kept separated from all mates.
Your veterinarian can supply you with pills that will reduce the odor of the urine during the heat period. There are also other items on the market to prevent the bloody discharge from staining household items or clothes, although most females in heat are very meticulous and tend to keep their genitals very clean.
Spaying is the permanent alternative for preventing a mating and should be considered if other preventive methods seem troublesome.