Female Dog Heat Cycles

Sexually mature female dogs (termed bitches) will go through a heat cycle once, or more commonly, twice a year. Each cycle consists of four stages:

Proestrus: Characterized by swelling of the vulva and release of a bloody vaginal discharge. Sometimes the discharge is milky and other times dark red (or somewhere in between in colour). Between bitches, there is a large variation in the length of proestrus, with an average length of 9 days, and a range of 1 to 17 days. This is the "pre-heat" during which the male is progressively more attracted to the female, but the female still rejects the male. This is a high female hormone (estrogen) stage.

Estrus: This phase is often termed standing heat because it is during this stage that the female will receive the male dog. Average length for estrus is 9 days, but again, between dogs there is large variation (range 3 to 21 days). At this point, the discharge may be absent, or perhaps straw coloured. This stage is characterized by an increase in progesterone hormone and an LH (luteinizing hormone) surge that leads to ovulation. Ovulation is the release of the eggs into the oviduct. If the eggs are healthy, they can then be fertilized if viable sperm are present.

Diestrus: At this point, the female usually no longer stands for the male, especially further into the diestrus stage. Vaginal discharge tapers off and the diestrus phase lasts an average of about 60 days. Diestrus begins 6 days after ovulation. Progesterone hormones remain high throughout this phase.

Anestrus: This is the resting phase between the active portions of the cycles. It normally lasts about 5 months.

Veterinarians can perform analysis of the cells of the vagina under the microscope (cytology) to determine the stage of estrus. There is a transition from non-cornified cells to cornified epithelial lining cells at the time of estrus. This reverses 6 days after ovulation.

Veterinarians can also measure progesterone levels in the bloodstream to determine what is happening with the heat cycle. From a base level of 2 nanograms per millilitre before LH release, it rises to 4-9 nanograms per millilitre at the time of ovulation. The progesterone hormone changes are used to help time mating.

Reprinted with permission from www.animalhealthcare.ca