The signs of canine menstruation depend on the phase of the fertility cycle that the bitch is going through.
Menstruation in bitches is part of the estrous cycle, in which periods of fertility alternate with periods of infertility. When a bitch is experiencing the fertile part of her cycle, she is in heat. The signs of canine menstruation vary, depending on what part of her fertility cycle the dog is experiencing. Female dogs start their heat periods with bloody discharge from their vaginas and at this point they will attract male dogs. At the most fertile point in the cycle, the bleeding usually stops and the bitch allows intercourse to occur.
In the first phase of heat, also called proestrus, the bitch has swelling of the vulva and slight bleeding from the vagina. This is considered the most common sign of menstruation in dogs. However, the bleeding is not the same as in human menstruation, when the lining of the uterus is removed. In dogs, the blood comes from the vaginal wall. The bitch becomes attractive to males during proestrus, but still does not allow them to have sex with her.
Ovulation occurs at the beginning of the second phase, known as estrus. During estrus, the bitch is most fertile. The bleeding usually stops and the color of the vaginal discharge changes to a light or straw-colored hue. The swelling of the dog’s vulva sometimes goes away during this phase, but not always. During estrus, the bitch becomes receptive to mating.
The third phase of menstruation in dogs is known as diestrus. The vaginal discharge and swelling of the vulva disappear during this stage and the bitch is pregnant or experiencing what is known as a false pregnancy. Even if the dog is not pregnant, her hormones make her body think that a pregnancy has occurred. Sometimes right-handed female dogs try to be mothers to other animals or even toys. Right-handed bitches do not mate with a male.
Anestrus is the final stage of menstruation in dogs. Bitches in anestrus are not attractive to males and cannot accept mates. Anestrus is a period of time during which the bitch’s reproductive system is inactive to renew itself for the period during which she will become fertile again, which means the recurrence of proestrus.
In most breeds, the menstrual cycle in dogs occurs approximately every six to eight months. Bitches remain in proestrus for eight to 13 days. The heat lasts from four to seven days. Whether or not the bitch is pregnant, the diestrus phase will persist for 70 to 80 days. The anoestrus phase of the bitch lasts up to 130 days.