Menstruation in dogs is part of the estrous cycle, during which periods of fertility alternate with periods of sterility. When a bitch knows the fertile part of her cycle, she is said to be in heat. The signs of canine menstruation differ, depending on what part of your fertility cycle a bitch is experiencing. Dogs begin their heat spells with a bloody secretion 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 dog allows intercourse.
In the first heat phase, also called proestro, the bitch feels a swelling in the vulva and bleeds slightly through the vagina. This is considered to be the most common sign of menstruation in dogs. Bleeding, however, is not the same as in human menstruation, when the lining of the uterus is lost. In dogs, the blood comes from the vaginal wall. The bitch becomes attractive to male dogs during procreation, but will not yet allow them to have sex with her.
Ovulation occurs at the beginning of the second phase, known as estrus. During estrus, the bitch is at her most fertile point. Her bleeding usually stops and the color of her vaginal flow becomes light or straw-like. Swelling of the dog’s vulva sometimes disappears during this phase, but not always. During estrus, the bitch becomes receptive to mating.
The third stage of menstruation in dogs is called right-handed. Vaginal discharge and swelling of the vulva disappear during this stage and the bitch is pregnant or is going through what is called a false pregnancy. Although the dog is not pregnant, her hormones make her body believe that a pregnancy has occurred. Sometimes right-handed dogs will try to be mothers of other animals or even toys. Dexterous bitches will not mate with a male.
Anestrus is the final stage of menstruation in dogs. Dogs in anesthesia are not attractive to male dogs nor can they accept a mate. The anestrus is a period of time during which the dog’s reproductive system falls asleep in order to renew itself during the time during which it will become fertile again, signified by the recurrence of the progesterone.
In most breeds, the menstrual cycle of dogs runs approximately every six to eight months. The dogs remain in the progester for eight to 13 days. The estrus lasts four to seven days. Whether the dog is pregnant or not, the dexterous phase will persist for 70 to 80 days. The anestrus phase of the bitch lasts up to 130 days.