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A female dog in heat will undergo several physical and behavioral changes throughout her heat cycle. Some common behaviors of a female dog in heat include receptiveness to male dogs and nervous or agitated behavior. She may also exhibit physical signs including bloody discharge and a swollen vulva.

Most dog owners who do not plan on breeding their dogs will choose to have their females spayed to prevent accidental pregnancy and hormone-related issues later in life. 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. Here's what to expect when your female dog comes into heat.

When Should I Expect My Dog's First Heat?

On average, most dogs will begin their first heat cycle between six and 15 months of age. 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.

What Are the 4 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.

1. Proestrus is the start of the heat period when your dog’s body is preparing to mate. Signs 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 is the mating phase where your female dog will be receptive to males. She may need to urinate more frequently than normal, as she's 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 toward other females.
3. Diestrus 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 is an inactive phase, and no signs of hormonal or sexual behavior are noticed.

What Signs Indicate 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 each phase of her heat cycle, you'll see her show specific behaviors as well as physical signs.

Common Behaviors of a Female Dog in Heat

Receptive to male dogs: Once your dog is in the midst of her estrus cycle, she will welcome the advances of male dogs. She will allow other dogs to mount her, may hump other dogs—including other female dogs—and may even try to mount your leg or other pets in the home.

Agitated, nervous, or aggressive behavior: In the first phase of estrus, your dog might seem skittery or nervous. She might become aggressive towards other household pets, or even towards you. As estrus progresses, she may become aggressive towards other female dogs but welcoming to male dogs.

Excessive licking of the genital area: Commonly, a female dog in heat will lick her genital region far more than usual.

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: At the beginning of the heat cycle, your female dog might keep her tail tucked close against her body, but as estrus progresses, you'll observe her holding her tail to the side, which alerts male dogs that she is ready for mating.

Physical Signs of Female Dogs in Heat

Swollen vulva: The opening to the dog's vagina is called the vulva and 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's a normal part of estrus.

Bloody or straw-colored discharge from the vulva: Your dog will have a discharge from her 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 or brownish color and diminishes in quantity.

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

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. Smaller 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 uterine infection called pyometra. A dog 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?

Veterinarians generally recommend spaying female dogs around six months of age to prevent mammary cancer, uterine infections, and other hormone-related issues. Some dogs are spayed even earlier, especially in shelter environments. However, current research suggests that large- and giant-breed dogs may need more time to grow before removing the hormones necessary for skeletal development. Talk to your veterinarian about your dog's unique needs before you decide what age is appropriate to spay your dog.