Domestic dogs complete one full cycle to the completion of ovulation per estrus (heat) cycle.  Depending on the breed and individual bitch, they can cycle one to four times per year.  Estrus is divided into four stages: proestrus, estrus, diestrus and anestrus.  Understanding the estrus cycle can assist you with timing the day of ovulation.

Proestrus is the beginning of the heat cycle, which is 7 to 9 days on average.  During this cycle, she will attract the male but is not receptive to mating yet.  Her vulva will start to swell and a reddish and serous discharge develops. The progesterone levels are low at this time.  Red blood cells, white blood cells, and normal vaginal epithelial cells are seen under the microscope. 

The next phase of the heat cycle is called estrus, which can range from 1 to 3 weeks in duration. Estrus is when the bitch is receptive to the male.  The estrogen levels begin to drop and the progesterone levels begin to rise.  There is a lighter pink discharge and the vulva is still swollen.  When the eggs from the ovaries are ovulated and have become mature, the estrus cycle is complete. 

Diestrus is the third phase of the estrus cycle, and can last up to 4 months.  The bitch is no longer receptive to the male and her vulva returns to normal with no visible discharge.  During this phase, progesterone levels are at their highest concentration.  At the end of this period, mammary development occurs prior to parturition, or whelping of the puppies. 

The last phase of the heat cycle is anestrus, which is the longest period lasting up to 4 months or more.  The bitch is not receptive to mating and the progesterone and estrogen levels are at their lowest.  This phase is considered to be the inactive stage of the canine estrus cycle.

It is important to determine the day of ovulation when trying to manage the breeding of a bitch who is difficult to get pregnant or to artificially inseminate. Diagnostic tests your veterinarian will use to determine the day of ovulation include vaginal cytology and checking progesterone levels.

When considering artificial insemination, ovulation timing is very important because optimal fertility is expected between 2 and 4 days after she has ovulated.  Monitoring the blood progesterone concentration is the best way to determine the time of ovulation.  The progesterone levels are very low during proestrus until the very late stages.  LH is a hormone that comes from the pituitary gland which aids the control of the estrus cycle.  At the very end of the proestrus cycle, there is an increase of LH which is known as the LH surge.  At this time, the surge causes the follicles on the ovaries to rapidly mature and become ready to ovulate.  Canines typically ovulate between 2 to 4 days after the LH surge. 

It can take 48 to 72 hours for the canine eggs to ovulate and to go through a maturation phase.  The optimal time to artificially inseminate the bitch is 2 to 3 days after the progesterone levels have increased.