While dogs may seem to have a menstrual cycle that is very similar to a woman's menstrual cycle, dogs do not experience an actual menstrual cycle. Instead, dogs have what is called an Estrus Cycle (or Heat Cycle), and the estrus cycle has its own unique phases. Sexually mature female dogs will go through a heat cycle once, or more commonly, twice a year. Each cycle consists of four stages: proestrus, estrus, diestrus, and anestrus
The Estrus Cycle
The estrus cycle of a dog can take 6 – 8 months from start to finish, but some breeds of dogs have a cycle which lasts one year. How long, or short, a dog's estrus cycle is depends on the dogs age, health status, and breed. A dog's estrus cycle will continue for most of the dog's life because, unlike primates, dogs do not go into a menopausal phase when they grow older. Instead, the length of the estrus cycle gradually increases as a dog ages.
The first phase of the estrus cycle begins with proestrus and lasts approximately 9 days. The dog's vaginal area becomes swollen and excretes a bloody discharge during this phase. This is also the phase that is often confused with a menstrual cycle. During this phase the dog's body prepares itself for a possible pregnancy, but the female will not accept the male at this time.
The second estrus phase also lasts approximately 9 days and this phase is called estrus. During this phase the female ovulates, and she will readily accept a male.
The third phase of the estrus cycle is called diestrus, and this phase can last between 70-80 days. The diestrus phase will begin regardless of whether the female dog becomes pregnant or not. During this phase the dog's body is 'hormonally pregnant', and this phase may also appear as a false pregnancy. Female dogs that are not pregnant may begin to 'mother' other animals or toys at this time, and they will not accept a male.
Anestrus is the last estrus phase. This phase can last for 130 days, and during this phase the females will not accept a male. This is a 'resting' phase where the dog's body repairs itself and starts to ready itself for another estrus cycle.