Vet Reviewed

By Carmela Ciuraru | September 27, 2023

What does “being in heat” mean, exactly? Simply put, it’s the time in a female dog’s cycle when she can get pregnant. The exact frequency and duration will depend on your dog’s breed, age, and size, but on average, dogs are in the mating period of heat, known as estrus, for one to two weeks. 

Unspayed female dogs go into heat about once every six months—and they’ll go into heat at certain intervals for their entire lives. If your dog’s cycles are dramatically inconsistent, talk with your vet. 

We can help you understand the different stages of heat and what to expect. The first two stages, proestrus and estrus, are when physical changes and mating behaviors occur.

What are the four stages of a dog’s reproductive cycle?

Proestrus: This is the beginning phase of heat, which lasts about a week. Your dog will start attracting the attention of male dogs, and at this stage you’ll see some swelling and bleeding from the vulva.

Estrus: This is the mating period of heat, lasting five to ten days. At this time, you’ll see a reduction in bleeding.

Diestrus: This period has a wide-ranging timespan; it can last anywhere from ten days to nearly five months. Your dog may be pregnant at this time.

Anestrus: This is a kind of downtime period before the next heat cycle begins.

The first heat cycle really varies by breed: toy breeds may come into heat as young as four months, while large breeds might not have their first heat cycle until they are two years old. Typically, however, most dogs have their first heat cycle between the age of six months old to fifteen months old.

What are the signs of a dog in heat?

By knowing what to expect when your dog is in heat, you can better support her through the four distinct stages. Here are some signs to look for:

  • Swollen vulva
  • Discharge (typically bloody) from the vulva
  • More frequent need to urinate (a signal to male dogs in the area that she is receptive to mating)
  • Behavioral changes, such as restlessness, nervousness, and mounting other dogs

When your dog is in heat, she might have a decrease in appetite and less energy than usual, or even seem agitated or aggressive at times—especially toward female dogs. This is all normal.

At the beginning of the estrus cycle, it’s normal for your dog to leave blood stains or spotting in her bed, on any furniture where she likes to sit, or on the floor. By knowing to expect this ahead of time, you can prepare accordingly and cover furniture with towels, or limit the rooms where your dog can be, keeping her off carpets and rugs. Just remember to be gentle and patient with her—stains can always be cleaned up. 

Also in the beginning of heat, you might observe changes in your dog’s tail position: either tucked close against her body, or tilted to the side (another alert to male dogs that she is ready to mate).

In the diestrus stage, either your dog will be pregnant, or her body will return to normal (the vulva swelling will go down) and there won’t be any more discharge.

During the inactive anestrus phase, mating behaviors typically go away, or are dramatically reduced, and her hormones will remain at normal levels. Although heat cycles will vary among dogs, most dogs have an average of two heat cycles a year. (Small breeds may have three heat cycles a year, however, and for giant breeds, it might happen only once every twelve months.) 

What should I do to help when my dog is in heat?

Apart from being loving and attentive toward your dog when she is in heat—especially the first time it happens—there are some practical steps you can take. 

It’s a good idea not to leave your dog alone outside in the yard. (A female dog ready to breed sends out powerful pheromones that attract all kinds of male dogs.) Similarly, while your dog is in heat, you should keep her on a leash at all times. Even if you’re used to giving her fun off-leash time at the park each day, while she’s in heat, she should always be on a leash. That’s because she will likely not have the same recall discipline that she usually does, and might run off with a male dog and not come back, or she might be unable to find her way back. Give yourself some peace of mind by ensuring that her ID tags and microchips are up-to-date with all your current contact information.

Finally, if you’re not planning to breed your dog, you might want to chat with your vet about spaying your dog after the first heat cycle is over. This is a personal decision, but it’s perhaps worth discussing with your vet and sharing any health or behavioral concerns you may have.