The heat cycle, better known as the estrus cycle typically occurs when a female dog reaches 6 months. However, there are many variants depending on the breed of the dog and its level of development. It is not unheard of for large breed dogs such as Saint Bernards and Mastiffs to go into heat at one year of age or even as late as 18 months.
The heat cycle in dogs is divided into three different phases which when combines together average about 21 days. The first phase is called Pro-estrus. This phase is characterized by vaginal swelling and vaginal bleeding. At this stage male dogs will appear interested however, the female dog will not allow them to mount. This stage usually lasts an average of 7 to 10 days.
The following stage is called estrus. At this point the female is actually fertile. The pink-red bloody discharge at this point is replaced by a straw colored discharge. At this point she will appear more interested in the male, and after some flirting, will keep her tail to the side and allow the male to mount. Should you witness a "tie" where the male and female are stuck together to each other, the chances of pregnancy are pretty high, but a tie is not a guarantee of pregnancy and some dogs who don't tie can also get pregnant. This phase lasts an average of 4 to 13 days.
The last stage is the final and it is called Diestrus. Even though the male will still attempt to mount, the female is no longer interested and is no longer fertile. This concludes the estrous cycle.
Heat cycles usually occur twice a year or once a year. However, in some cases, the dog's heat may go unnoticed. This may occur because the dog may tend to clean herself up quite a bit licking away the vaginal discharge. Even though not that common, some dogs may go through what is called a "silent heat". In this scenario, the dog exhibits subtle symptoms which the owner will not notice. However, male dogs may be great detectors of such silent heats and may confirm them by observing their behavior.
However should your female dog appear not to go in heat for some time, it may be helpful to report to your vet to rule out some conditions that may suppress the heat cycle such as hypothyroidism,diabetes mellitus, cancer Cushing's disease and Addison disease.
If you are considering breeding your dog, the estrus phase is when you want her to mate. If you are not planning to breed the estrus phase is when you want to keep her safely inside far from male dogs. Males can detect females in heat from many miles away. So do not be surprised if you may find some male dogs waiting behind your door.