If your female dog has not been spayed, she goes through menstruation, known as the estrous cycle. This is commonly referred to as being in heat, or in season. When a female dog is in heat, she can get pregnant. During this cycle a female dog bleeds from her vagina and often has swelling of the vulva. She also urinates more frequently.
Purchase diapers or pants designed for use during menstruation if your dog's bleeding is heavy enough to make a mess. You can also put a pair of men's or boy's underwear on her. Put the underwear on backwards and put the dog's tail out through the fly. If bleeding is heavy, affix a female pantyliner or menstrual pad inside of the underpants.
Confine the dog to one room, with a floor that is easy to clean, if she refuses to wear underpants. Use a baby gate to close her in the room when the bleeding is heaviest.
Put the dog in a crate at night during menstruation. The floor of the crate is easy to clean in the morning.
Stay with your dog whenever she is outside during menstruation. Do not leave her outside alone, even in a fenced in yard. Male dogs can smell when a female is in heat and can find a way to get to her, even if they have to climb over, or dig under a fence.
Use care when walking your dog during menstruation. If there are male dogs in the area, put her in the car and drive to a remote place to go for a walk. If you walk her around the neighborhood while she is in heat, she leaves a trail of scent for male dogs to follow her home.
Take your dog outside more frequently while she is menstruating to allow for the more frequent urination. Do not get upset with her if she has an accident inside the house.
Most dogs menstruate for 21 days, but this varies from dog to dog and from season to season, so take extra precautions against pregnancy for 4 weeks instead of 3. The average female dog goes through menstruation twice a year, or every 6 months. Again this can vary from one dog to the next. If you are not going to breed your female dog, or if she is done having litters, get her spayed. An unspayed dog has a greater risk of several different health problems, including pyometra, a uterine infection that can be fatal.