Female Heat (estrous) Cycle:

Most dogs come into heat twice per year, or about every 6 months. The interval can vary from patient to patient. Small dogs may cycle up to 3 times per year and giant breeds may cycle once every 12 months.

Canine Estrous (reproductive) Cycle is made up of 4 different stages and each stage has differing signs related to behavior, physical or clinical changes, hormonal changes, physiological changes, and cytologic (vaginal smear) changes.

  1. Proestrus is the stage that most owners start noticing changes and when we say “the dog is in heat”.  This stage lasts for an average of 9 days, but can range anywhere from 0-27 days.  This is when male dogs will be attracted to the female, but she will not be receptive.  The estrogen levels will peak and the follicles will develop.  The vulva will usually be swollen with a blood-tinged discharge.  Vaginal cytology will show mixed types of cells, often with red blood cells.
  2. Estrus is the stage when the female is receptive to the male.  This stage will last an average of 9 days based on behavioral signs, but can range from 4 to 24 days.  The fertile period occurs during this time.  The vulva is enlarged, but softens a little.  There is a decrease in the blood in the vulvar discharge.  Estrogen levels are dropping and progesterone levels are starting to increase.  Vaginal cytology shows predominately cornified (flattened) epithelial cells.  (Ideal time for breeding).
  3. Diestrus is the stage following estrus and the female is no longer receptive to the male.  This stage lasts for about 2 months.  Estrogen levels are low, while progesterone peaks 3 to 4 weeks after the start of diestrus and then declines to basal levels by the end of diestrus.  This increase and then decrease in progesterone will occur regardless of if the dog is pregnant.  Cytology shows a shift back to basal cells, with fewer red blood cells than in proestrus.
  4. Anestrus is the time between diestrus and the next proestrus.  This stage will last for about 4 months, though certain breeds can be much longer.  The vulva is no longer swollen, there is no vaginal discharge.  The body uses this time to allow the uterus to prepare for the next possible pregnancy.  Cytology shows basal cells.

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