The canine estrous cycle is made up of four stages namely proestrus, estrus, diestrus, and anestrus. Each of these stages is characterized by unique signs related to behavioral, physical, clinical, physiologic, hormonal, and cytologic changes.
Different dog breeds may show varying signs and the duration of each stage can also vary. However, the four stages are universal and carry the following notable symptoms.
Stage 1: Proestrus
This is the first stage when a female dog is in heat and lasts up to a maximum of 27 days, with the average being 9 days. During this stage, estrogen levels will rise, follicles will develop and the vulva will begin to swollen. You will also notice blood-tinged vulvar discharge.
If vaginal cytology is performed, results will indicate the presence of different types of blood cells, including red blood cells. It’s vital to note that during the proestrus stage, male dogs will be attracted to the female dog in heat but the bitch will not be receptive and can even be overly aggressive and agitated.
Stage 2: Estrus
This is the stage where the female dog in heat is ready to mate and is receptive to male dogs. It lasts for about 4 to 23 days, with the average being 9 days. The vulva will stage be enlarged but soft with decreased blood-tinged discharge. During this stage, estrogen levels will be dropping while progesterone levels will rise.
The rising levels of progesterone will lead to some common behaviors of a female dog in heat. The habits include mounting and humping other dogs, being receptive to male dogs sniffing, frequent urination around male dogs, and postural and tail position changes. Vaginal cytology will show predominantly flattened epithelial cells.
Stage 3: Diestrus
The third stage is called diestrus and the female dog is no longer receptive to male dogs. The stage can last for up to two months and estrogen levels will remain low. On the other hand, progesterone levels will peak 3 to 4 weeks after the start of this phase then decline to the basal level as the stage comes to an end.
The rise and decline of the two hormones will occur regardless of whether the female dog achieved successful mating (i.e. became pregnant) or not. Performing vaginal cytology will show fewer red blood cells than in the previous stage.
Stage 4: Anestrus
This is the final phase of a canine estrus cycle and lasts for about 4 months before the proestrus phase sets in again. The swelling of the vulva will be gone and so will the vulvar discharge. During this stage, the female body will be preparing and the uterus getting ready for the next possible pregnancy. Cytology analysis will indicate the presence of basal cells.
In case you’re not sure if your canine friend is about to mate, here are a few pictures of a female dog in heat to help you out. Keep in mind that the duration for each of these stages varies, with smaller dog breeds having shorter periods compared to large breeds. Small dog breeds can also experience their first heat cycle within 6 months after birth while it can take the large breeds up to 18 months.