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A female dog's menstruation, more commonly referred to as estrus, occurs when she reaches puberty and is able to become pregnant with puppies. During estrus, female dogs undergo a variety of behavioral, physical and hormonal changes.
There are two stages of a female dog's reproductive cycle that occur before the actual menstruation: anestrus and proestrus. During anestrus, neither males nor females are interested in each other. The external female reproductive organ known as the vulva is not yet swollen or leaking discharge, the vaginal walls are thin and hormones are at their baseline amounts. Anestrus lasts for between 90 and 150 days, depending on the breed and size of the dog. Proestrus is when males begin to become interested in females, while females remain uninterested in males. This is because the female dogs have increased hormone (estrogen) levels. This increase in hormones leads to a swollen vulva, bloody discharge secretions and water retention in the vaginal walls. Proestrus lasts three to 17 days.
The Main Event
Once discharge changes from bloody to watery and pink in color, the female dog is experiencing the estrus stage. This stage is also referred to as "being in heat," or when a female is fertile and becomes interested in mating with a male. During estrus, the vulva is soft, but still swollen, and females begin to partake in unusual behaviors. For example, females experiencing estrus urinate more often than females not in heat. Females may also urinate on objects inside or outside of your home to mark them. The urine contains hormones and pheromones, which lets neighboring male dogs know that she is ready to mate. Additionally, estrogen levels decrease while progesterone levels increase.
The Show Goes On
When ready to mate, the female's cervix relaxes and slightly opens. Female dogs begin to flag their tails, or move them from side to side, and lift their rumps in the air to get a male's attention. Females experiencing estrus may become aggressive with other female dogs. The estrus cycle ends when symptoms become less noticeable. The vulva becomes less swollen and discharge changes from pink to clear. Usually, estrus lasts for three to 21 days.
Closing Up Shop
When a female refuses male attention, she is experiencing the final part of her reproductive cycle known as diestrus. The vulva is returned to its normal size and discharge stops. Some females may act as though they are pregnant, even if they are not. This is known as pseudo pregnancy, where female dogs display behaviors such as nesting their toys or producing milk. Whether the female dog is pregnant or not, diestrus typically lasts for 60 days.