Menstruation in dogs is part of the estrus cycle, in which times of fertility alternate with times of infertility. When a female dog is experiencing the fertile part of her cycle, she is said to be in heat. Signs of canine menstruation differ, depending on what portion of her fertility cycle a female dog is experiencing. Dogs begin their heat periods with bloody discharge from their vaginas, and at this point, they will attract male dogs. At the most fertile point of the cycle, the bleeding usually stops, and the female dog allows intercourse to occur.
In the first phase of being in heat, also called proestrus, the female dog experiences swelling in her vulva and bleeds lightly from her vagina. This is taken as the most common sign of menstruation in dogs. The bleeding, however, isn't the same as in human menstruation, when the lining of the uterus is shed. In dogs, the blood comes from the vaginal wall. The female dog becomes attractive to male dogs during proestrus, but she will not allow them to have intercourse with her yet.
Ovulation occurs at the beginning of the second phase, known as estrus. During estrus, the female dog is at her most fertile point. Her bleeding usually stops, and the color of her vaginal discharge changes either to clear or to a hue similar to that of straw. The swelling of the dog's vulva sometimes goes away during this phase, but not always. During estrus, the female dog becomes receptive to mating.
The third phase of menstruation in dogs is referred to as diestrus. Vaginal discharge and vulva swelling both go away during this stage, and the dog is either pregnant or experiencing what is known as a false pregnancy. Even if the dog is not pregnant, her hormones trick her body into thinking that a pregnancy has occurred. Sometimes dogs in diestrus will try to be mothers to other animals or even to toys. Female dogs experiencing diestrus will not mate with a male.
Anestrus is the final phase of menstruation in dogs. Female dogs in anestrus are neither attractive to male dogs nor can they accept mates. Anestrus is a period of time during which the reproductive system of the dog lies dormant in order to renew itself for the time during which she will again be fertile, signified by the recurrence of proestrus.
In most breeds, the cycle of menstruation in dogs runs its course roughly every six to eight months. Female dogs stay in proestrus for eight to 13 days. Estrus lasts anywhere from four to seven days. Whether or not the dog is pregnant, the diestrus phase will persist for 70 to 80 days. The anestrus phase of the female dog lasts for up to 130 days.