The Heat Cycle in Female Dogs: What Is Normal and When to See a Vet

The four stages of the cycle

Vets divide the cycle into four stages. Owners mostly notice the first two, which together make up what people call heat.

• Proestrus. The vulva swells and a bloody discharge appears. Male dogs are drawn to her, but she does not accept them yet. This stage lasts about nine days on average.
• Estrus. She becomes willing to mate and may stand still or move her tail aside when a male comes close. The discharge often turns lighter, pink or straw-colored. This stage also averages about nine days, and she ovulates during it.
• Diestrus. Her hormones shift, she stops accepting males, and the vulva slowly returns to normal size. This stage lasts about two months or a little longer, whether she is pregnant or not.
• Anestrus. A resting phase of several months while the uterus recovers before the next cycle.

Other common signs during heat include peeing more often, marking with urine, licking the genital area, restlessness, and more interest in male dogs. Some dogs keep themselves so clean that their owners see very little blood.

How long heat lasts and how often it comes

The bloody discharge usually lasts about two to three weeks. Each stage varies a lot from dog to dog. A proestrus of only a few days, or one of close to three weeks, can still be normal.

Most dogs come into heat every five to eleven months, and about seven months is the average. That works out to roughly twice a year. Small breeds may cycle three times a year. Some giant breeds, and breeds such as the Basenji, may cycle only once a year.

The first heat usually arrives between 6 and 24 months of age. Small breeds tend to start earlier, and large and giant breeds later. Cycles are often irregular in young dogs, and it can take up to two years for a steady pattern to settle in. Writing down the start and end dates of each heat makes it easier for your vet to spot a problem later.

Unusual cycles that can still be normal

A split heat starts with swelling and bleeding, then fades before she ovulates. A full heat usually follows within one to two months. Split heats are most common in a dog's first cycle and are often harmless. If they keep happening, ask your vet to check her.

A silent heat is a cycle with very little swelling or discharge, even though her body goes through the normal stages. The most common reason is simply that nobody noticed. Male dogs often still show interest.

Some young dogs, more often in large breeds, develop swelling of the vaginal lining during heat. A pink or red mass of tissue may bulge out of the vulva. It tends to shrink once the heat ends, but it usually comes back with each cycle. Ask your vet to examine it.

When heat lasts too long

Bleeding that goes on well past three weeks is a good reason to call your vet. When signs of heat last about six weeks or more, vets usually call it persistent heat. It happens when estrogen keeps acting on her body for too long.

A common source is a follicular cyst, a fluid-filled sac on an ovary that does not release its egg and keeps making estrogen. Less often, an ovarian tumor that produces estrogen is to blame. A dog can also take in estrogen from outside, for example by licking or rubbing against skin where a person has applied a hormone cream, gel, or spray. In spayed dogs, this kind of exposure can cause a swollen vulva and discharge that look just like heat.

Long exposure to estrogen can harm the bone marrow, where blood cells are made. This can lead to anemia, bleeding problems, and infections, and the damage can be permanent. Too much estrogen can also cause hair loss on both sides of the body, often starting around the genital area, and darker, thicker skin. A long heat can also make pyometra, a serious infection of the uterus, more likely later.

When it is an emergency

Some signs need a vet the same day. Call your vet or an emergency clinic right away if you notice any of these.

• Pus-like, bad-smelling, or brown discharge, especially in the weeks after a heat has ended
• A swollen or tight belly, vomiting, or loss of appetite
• Drinking and peeing much more than usual
• Weakness, collapse, pale gums, or small bruises on the skin or gums
• Heavy bleeding that soaks bedding quickly or does not slow down
• Fever, or a dog that seems dull and unwell

Pyometra most often appears one to two months after a heat, although it can develop anywhere from a few weeks to about four months after it. Without prompt treatment it can quickly become life-threatening, and if the cervix is closed, there may be no discharge at all, only a dog who is getting sicker. For the signs and treatment in full, see pyometra in dogs.

How vets look into it

Your vet will ask when the heat started, how earlier cycles went, whether she could have mated, and whether anyone at home uses hormone products. A physical exam comes next. A common test is a vaginal smear, where a swab of cells is checked under a microscope to show the stage of the cycle and how strongly estrogen is acting on her body. Blood tests can measure progesterone, which shows whether she has ovulated, and a complete blood count checks for anemia and other signs of bone marrow trouble. An ultrasound can show the ovaries and uterus, although small cysts are not always easy to see.

Treatment depends on the cause. For persistent heat, spaying to remove the ovaries and uterus is usually the preferred and most lasting option. For a dog kept for breeding, a vet may first try hormone treatment to make her ovulate, then watch her closely for pyometra afterward. When the estrogen came from a hormone product in the home, signs often fade once the exposure stops. Your vet will explain which options fit your dog.

Care during heat and prevention

A normal heat needs no medical treatment, and a few habits help keep her safe. It is best to keep her on a leash on walks, stay with her whenever she is outside, and keep her away from males that are not neutered, since they may try hard to reach her. Skipping dog parks until the heat is fully over is wise, because she can become pregnant at any point during estrus. Washable bedding or dog hygiene pants help keep the house clean. If an accidental mating happens, contact your vet soon to talk about options.

Spaying ends heat cycles and prevents pregnancy and pyometra. It can also lower the risk of mammary tumors. Many vets prefer to wait about two to three months after a heat before spaying, because the blood supply to the uterus is larger during and just after heat. The best age and timing vary from dog to dog, so ask your vet what fits yours. Our guide to the pros and cons of spaying a female dog covers the benefits, risks, and timing in more detail.

If you adopted a female dog

With an adult rescue dog, her reproductive history is often unknown. At Paw Squadron, every animal gets a vet checkup, parasite treatment, vaccinations, and a microchip before adoption. Ask for her medical records and check whether they say she is spayed. If you are unsure, your vet can help you find out. A heat after adoption usually means she still has her ovaries.

If a dog known to be spayed shows signs of heat, one possible cause is ovarian remnant syndrome. A small piece of ovary was left behind during surgery and still works. Signs can appear months or even years after the spay. Vets can confirm it with swabs and blood tests, including a hormone test that detects ovarian tissue, and treat it by removing the leftover tissue. Hormone creams or sprays used by people in the home are another cause to rule out.

Bloody discharge is not always heat. Canine transmissible venereal tumor is a cancer that passes between dogs during mating and through licking or sniffing. In females it can cause bleeding from the vulva that is easy to mistake for heat. It is rare in Northern and Western Europe but still found in parts of Southern and Eastern Europe, mainly where many dogs roam free. It usually responds well to treatment. If the bleeding seems unusual, tell your vet where your dog came from.

For more guides on caring for an adopted dog, browse PawPedia or read our FAQ.