In veterinary medicine, we do not refer to dog’s cycles as menstruation like in humans. Instead, dogs go throughestrous cycles, and these cycles are broken down into four phases, each lasting different amounts of time:
Anestrus – the longest phase; the time of least reproductive/hormonal activity (between heats)
Proestrus – estrogen levels peak and the vulva enlarges; usually lasts 9 days
Estrus – the most fertile time for breeding, when the female is receptive to males; lasts 9-14 days
Diestrus – female no longer receptive and progesterone peaks, regardless of being pregnant or not being pregnant; lasts for 2 months
Owners typically see signs that their dog is going into estrus or “heat” when it is really proestrus that is starting. Your dog’s vulva may appear enlarged, and you may begin to see bloody vaginal discharge.
These female dogs may not be receptive to male dogs yet, and they may have an increased appetite or a change in their personalities. Dogs in proestrus may also seem nervous, and they may urinate more frequently in order to signal male dogs for their readiness via hormones and pheromones present in her urine.
In true estrus, which is when a dog is ready to be bred, the vulva begins to shrink and the bloody discharge may start to have a pinkish, watery appearance. Your dog may be more receptive to male dogs at this time, and you may notice her start to plant her feet on the ground and raise her tail high into the air or move her tail to the side. This position is known as “flagging.”
Diestrus occurs directly after estrus and lasts for about two months. Progesterone, which is the hormone that is responsible for maintaining a pregnancy, is present in high amounts, even if an actual pregnancy did not take place. This may explain why some female dogs can go through “false” pregnancies where they gain weight, eat more, and can appear pregnant but aren’t really.
Anestrus is the time period when your dog is not experiencing any of these cycles. The average female dog can go into heat or estrus twice a year. Small breed dogs can go into heat as often as every four months while large and giant breeds dogs can go up to two years before they have a heat cycle. Basenjis and Tibetan Mastiffs are known for having an estrus cycle only once a year.