7 Signs Your Dog Is in Heat

Learn the common behaviors of a female dog in heat

two dogs playing and running

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When a female dog is "in heat," it means that the dog is ready to breed and become pregnant. The proper term for this stage of a female dog's reproductive cycle is estrus. While most dog owners who do not plan on using the dog for breeding choose to have their female dog spayed, if you have recently adopted an intact female dog or do want to breed the dog, you should know what to expect when your dog comes into heat.

A dog in heat may exhibit signs including bloody discharge, receptiveness to male dogs, agitated behavior, and much more. An unspayed female dog going through a heat cycle will experience different symptoms in each of the four phases. A bloody discharge is the most common sign a dog is in heat, but it's not the only sign indicating your pooch is ready to mate. Having an unspayed dog—especially if you also have an intact male dog—in your home can be a challenge, but knowing what to expect can help prevent problems from arising.

When Should I Expect My Dog's First Heat?

The age of a dog’s first heat cycle varies greatly between breeds. Toy breeds can come into heat for the first time as young as four months, while large and giant breeds may be as old as two years before experiencing a first heat cycle. On average, most dogs will have their first heat cycle between six and 15 months of age.

What Signs Indicate That My Dog Is in Heat?

The more aware you are of your dog's cycle, the more prepared you will be for any physical and behavioral changes that may occur during her heat. During each phase of her heat cycle, you will notice different changes, and they may include the following seven signs:

  • Swollen vulva: The opening to the dog's vagina is called the vulva. It is located right below the dog's anus. When in heat, your dog's vulva will swell and redden. This can be very pronounced and alarming to dog owners seeing it for the first time, but it a normal part of estrus.
  • Bloody or straw-colored discharge from the vulva: Your dog will have a discharge from its vulva while in heat. At the beginning of estrus, the discharge is typically bloody, and it may leave blood spots or stains on the dog's bedding, your furniture, or the floor. As estrus continues, the discharge generally becomes more of a yellowish color and diminishes in quantity.
  • Receptive to male dogs: Once your dog is in the midst of the estrus cycle, it will welcome the advances of male dogs. Your dog will allow other dogs to mount her, may mount other dogs—including other female dogs—and may even try to mount your leg or other pets in the home.
  • Excessive licking of the genital area: Commonly, a female dog in heat will lick the genital region far more than usual.
  • Agitated, nervous, or aggressive behavior: In the first phase of estrus, your dog might seem skittery or nervous. It might be aggressive towards other household pets, or even towards you. As estrus progresses, your dog may be aggressive towards other female dogs, but welcoming to male dogs.
  • Urinating more frequently: Dogs in heat tend to urinate frequently as a method of alerting male dogs in the area that they are receptive to mating.
  • Change in tail position: In the beginning of the heat cycle, your female dog might keep its tail tucked close against the body, but as estrus progresses, you'll observe your dog holding its tail to the side, which alerts male dogs that she is ready for mating.

With clues gleaned from your female dog’s physical appearance and behavior, you can determine which stage of her heat cycle she is experiencing.

Signs your Dog is in Heat

The Spruce / Lara Antal

What are the Four Stages of the Canine Heat Cycle?

During your dog’s heat cycle, she will experience four phases, noted by various changes in her body and behavior. The four stages of the canine heat cycle are as follows:

  1. Proestrus: Proestrus is the start of the heat period where your dog’s body is preparing to mate. Signs seen during this phase include a swollen vulva, blood-tinged discharge, excessive licking of the genital area, clingy behavior, and aggression toward male dogs. Your dog may also hold her tail close to her body.
  2. Estrus: The estrus phase is the mating phase where your female dog will be receptive to males. You may notice that your dog seems to be urinating more frequently than normal, as she is marking spots to indicate her readiness to breed. Although she may be leaving urine marks in areas, her vaginal discharge will slow and may change to a straw color. Since your dog is ready to mate, she will approach males with her tail held to the side but may be aggressive towards other females.
  3. Diestrus: This phase occurs directly after the “in heat” stage and allows your dog’s body to either return to normal or develop into a pregnancy. Her vulva will return to normal size and vaginal discharge will disappear.
  4. Anestrus: Anestrus is an inactive phase, and no signs of hormonal or sexual behavior are noticed.

How Often Will My Dog Go into Heat?

Dogs have an average of two heat cycles per year, roughly six months apart. Some females will have irregular cycles, especially if they are very young or very old. Small breeds may cycle three times per year, while giant breeds may only cycle once every 12 months. Unlike some other species, canine estrous cycles are not dependent on the seasons, sunlight, or temperature.

What Should I Do if My Dog Is in Heat?

If your dog is experiencing her first heat cycle, it can be an unsettling situation for both of you. Follow these tips to ensure her heat goes as smoothly as possible:

  • Never let your dog out in the yard alone. Do not underestimate a male dog’s drive to find a female who is emitting breeding pheromones. You may walk outside to find a strange male dog tied to your female.
  • Never let your dog off her leash when she’s in heat. Although your dog may have excellent obedience skills, her recall ability may fall by the wayside when she’s influenced by her hormones and is intent on finding a male.
  • Ensure your dog’s ID tags and microchip information are up-to-date. If the unthinkable happens and your dog escapes from your yard or runs off, ensure you can be reunited with legible, updated ID tags and current microchip contact info.
  • Consult your veterinarian if you notice signs of illness. Occasionally, a female dog can experience health issues after a heat cycle when the uterine lining remains thickened and produces more fluid, creating the ideal environment for bacterial growth. This can lead to a life-threatening pyometra, or uterine infection. A pet with a pyometra may drink or urinate excessively, produce thick vaginal discharge, or have a decreased appetite, abdominal pain, vomiting, or lethargy.
  • Consider spaying your dog after her heat cycle is over. If you have no plans to breed your dog, consider waiting until after her heat cycle is over to spay her. Your veterinarian can advise you on the appropriate age to spay your pet, and will likely recommend that you wait until she is finished with her estrus phase to spay her.

At What Age Should I Spay My Dog?

Although veterinarians used to recommend spaying your dog as young as four months old to ensure she never experienced a heat cycle to prevent mammary cancer, current research is leaning toward allowing large- and giant-breed dogs to grow before removing the hormones necessary for skeletal development. Discuss health concerns with your veterinarian before deciding what age is appropriate to spay your dog.

  • How long does a dog stay in heat?

    Dogs are in heat for a week-and-a-half to two weeks. Estrus can last as long as four weeks in some dogs. However, your dog will not bleed or have discharge for the entire heat cycle.

  • Can a dog get pregnant when not in heat?

    No, she cannot. Dogs can only get pregnant when they are in heat. While pregnancy is possible during the entire heat cycle, the dog is most fertile around a week or so after estrus begins. The window of highest fertility is around five days long.

  • How do you keep the house clean when a dog is in heat?

    The best way to keep your house clean is to outfit your dog in a heat diaper, something that will catch the blood that she is discharging. It's also helpful to keep the dog off furniture and carpeted areas during this time, or spread towels over areas your dog frequents.

If you suspect your pet is sick, call your vet immediately. For health-related questions, always consult your veterinarian, as they have examined your pet, know the pet's health history, and can make the best recommendations for your pet.
Article Sources
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  2. Sato, Junko et al. Comparative Histopathology Of The Estrous Or Menstrual Cycle In Laboratory AnimalsJournal Of Toxicologic Pathology, vol 29, no. 3, 2016, pp. 155-162. Japanese Society Of Toxicologic Pathology, doi:10.1293/tox.2016-0021

  3. Estrus And Mating In DogsVCA Animal Hospitals, 2020.