The canine reproductive
cycle can be described as the “sexual season” of the
bitch. It may also be referred to as a
bitch “being in heat”.
The bitch will enter puberty on average between 5-24 months of age. She will have a heat cycle on average of every 5-
12 months. The average cycle will last 21 days. In theory, most bitches do not reach their maximal reproductive
potential until their third or fourth estrus.
Identifying the beginning of the estrus cycle is by vulvular swelling and or vaginal discharge. The vaginal discharge will
range in color on average from brownish red to red to pinkish-red. Pre-breeding evaluations by the veterinarian is
recommended at day 5 for bloodwork and vaginal cytology.
There are four stages of the canine estrous cycle:
Proestrus
• Beginning of the cycle.
• The bitch will attract males but will not permit breeding.
• Blood tinged vaginal discharge.
• Swollen, turgid vulva.
• Average duration is 9 days, can range 0-17 days.
• Blood levels of the hormone estrogen rise, changing vaginal cells and peaks when acceptance occurs, then it
declines during estrus.
• Blood levels of the hormone progesterone remains low for the first few days, and then starts to increase 2-3 days
before acceptance of the male.
• Cells seen in a vaginal cytology: Variable amounts of RBCs and WBCs. Initially contains small round cells with a
large nucleus (non-cornified), as proestrus progresses WBCs decline and cells become cornified with little or no
nucleus.
Estrus
• Period of receptivity of the male after estrogen begins to fall. The dam is the most receptive to breeding during
this period.
• Average duration is 9 days, can range from 3-21 days.
• Physical characteristics of this cycle include “flagging” the tail to one side as the male approaches from the rear.
“Tilting” of the vulva in a dorsal direction upon stimulation by the male’s precopulatory behavior, and “standing” for the
male as mounting is attempted.
• Blood levels of the hormone progesterone and LH (luteinizing hormone) increase and give rise to ovulation.
• During ovulation the eggs will then take 2-3 days to mature, and will last 2-4 days on average.
• Cells seen on vaginal cytology: Almost all cornified, No WBCs, RBCs may or may not be present.
Diestrus/Metestrus
• By definition is the phase during which luteal ovarian tissue produces progesterone.
• Lack of receptivity to breeding.
• Sharp change in vaginal cytology (cells).
• Significant decreases and change in color of vaginal discharge.
• Blood levels of the hormone progesterone continue to rise until approximately day 15 of diestrus, after this will
gradually decrease for 5-6 weeks, going back to the base line levels.
• If bitch becomes pregnant, the progesterone level will remain high to maintain pregnancy.
• Cells seen on vaginal cytology: WBCs reappear, Increasing non-cornified.
Anestrus
• A quiet, non active phase in between heat periods. This is a period of regeneration. Usually last 4-5 months.
• Cells seen in a vaginal cytology: Non cornified, variable numbers of WBCs
bitch “being in heat”.
The bitch will enter puberty on average between 5-24 months of age. She will have a heat cycle on average of every 5-
12 months. The average cycle will last 21 days. In theory, most bitches do not reach their maximal reproductive
potential until their third or fourth estrus.
Identifying the beginning of the estrus cycle is by vulvular swelling and or vaginal discharge. The vaginal discharge will
range in color on average from brownish red to red to pinkish-red. Pre-breeding evaluations by the veterinarian is
recommended at day 5 for bloodwork and vaginal cytology.
There are four stages of the canine estrous cycle:
Proestrus
• Beginning of the cycle.
• The bitch will attract males but will not permit breeding.
• Blood tinged vaginal discharge.
• Swollen, turgid vulva.
• Average duration is 9 days, can range 0-17 days.
• Blood levels of the hormone estrogen rise, changing vaginal cells and peaks when acceptance occurs, then it
declines during estrus.
• Blood levels of the hormone progesterone remains low for the first few days, and then starts to increase 2-3 days
before acceptance of the male.
• Cells seen in a vaginal cytology: Variable amounts of RBCs and WBCs. Initially contains small round cells with a
large nucleus (non-cornified), as proestrus progresses WBCs decline and cells become cornified with little or no
nucleus.
Estrus
• Period of receptivity of the male after estrogen begins to fall. The dam is the most receptive to breeding during
this period.
• Average duration is 9 days, can range from 3-21 days.
• Physical characteristics of this cycle include “flagging” the tail to one side as the male approaches from the rear.
“Tilting” of the vulva in a dorsal direction upon stimulation by the male’s precopulatory behavior, and “standing” for the
male as mounting is attempted.
• Blood levels of the hormone progesterone and LH (luteinizing hormone) increase and give rise to ovulation.
• During ovulation the eggs will then take 2-3 days to mature, and will last 2-4 days on average.
• Cells seen on vaginal cytology: Almost all cornified, No WBCs, RBCs may or may not be present.
Diestrus/Metestrus
• By definition is the phase during which luteal ovarian tissue produces progesterone.
• Lack of receptivity to breeding.
• Sharp change in vaginal cytology (cells).
• Significant decreases and change in color of vaginal discharge.
• Blood levels of the hormone progesterone continue to rise until approximately day 15 of diestrus, after this will
gradually decrease for 5-6 weeks, going back to the base line levels.
• If bitch becomes pregnant, the progesterone level will remain high to maintain pregnancy.
• Cells seen on vaginal cytology: WBCs reappear, Increasing non-cornified.
Anestrus
• A quiet, non active phase in between heat periods. This is a period of regeneration. Usually last 4-5 months.
• Cells seen in a vaginal cytology: Non cornified, variable numbers of WBCs
