When female dogs enter puberty, they begin to have heat cycles. The dog's heat (estrous) cycle consists of four distinct stages: Proestrus and Estrus that both happen during visible bloody discharge and Diestrus and Anestrus, that take place after the discharge.
What most people refer to as “coming into heat” or “coming into season” refers to the timing when a female has bloody discharge from her vulva. This period has two stages called Proestrus and Estrus. While knowing that there are two parts of the heat makes absolutely no difference to a pet owner, it is significant to understanding the components of a dog’s estrous cycle for individuals planning to breed. It can also be helpful for those looking to plan when to have their dog spayed to prevent pregnancy or future heat cycles.
When do heat cycles start and how often do they occur?
If you look into many research resources, you will find that "dog’s first heat can start between the age of 6-24 months, with smaller breeds averaging earlier than larger breeds". We typically see first heat at around 12-14 months and second heat at 18-20 months. Most dogs will have two heats per year; the average time between heats is 6 months, however we have at least one dog that goes into heat every four months and one that goes into heat every nine months. The most notable sign of a dog’s heat cycle is bloody vaginal discharge, usually lasting between 14 and 21 days.
The four estrous cycle stages include proestrus, estrus, diestrus, and anestrus.
What is happening during this stage: the estrogen produced in this stage causes bloody vaginal discharge and swelling of the vulva (or external genitalia). Dogs will be attractive to and may demonstrate playful behavior with male dogs during this stage but are not yet receptive to breeding. This stage can last, on average, between seven to ten days. Dogs are not yet fertile at this stage, however they may get pregnant later if mating happens during this time, as sperm will survive in vaginal cavity for up to ten days!
What is happening during this stage: Estrus begins when the female is has ovulated and is receptive to breeding. Increased progesterone hormonal changes are what influence the female's receptivity to breeding. The vaginal discharge will change from bloody red to clear pink with some blood . Estrus usually lasts two to five days.
What is happening during this stage: Diestrus begins when the female no longer shows signs of estrus, such as standing to be mounted. Progesterone (the pregnancy maintenance hormone) will increase during this stage, whether a dog is pregnant or not. Diestrus ends when progesterone concentrations return to baseline. During this stage, female dogs will no longer be attractive to males, nor will they allow mating. The external genitalia is indistinguishable between diestrus and the next stage, anestrus. Typically this stage lasts 60 days.
What is happening during this stage: Anestrus is the stage a dog enters either at the end of her heat cycle or after having a litter. Progesterone levels remain low throughout this stage. There is a period post-partum or after a normal heat cycle where the uterus must undergo a process called involution. This process repairs the uterus to prepare for repeating the estrous cycle and requires about five to seven months to complete.