Here’s what you should know about the dog heat cycle—and what to expect if your pup isn't spayed.
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dog looking longingly into the camera; how long are dogs in heat?
Ryan Murphy / Getty
| Credit: Ryan Murphy / Getty

If your sweet little girl isn't spayed, don't be caught off guard if she gets her period. This is the beginning of her estrus cycle, commonly called "heat." During heat, a female dog is open to mating and can become pregnant. And while bleeding is one of the most obvious signs a dog is entering heat, her estrus cycle actually lasts much longer than this initial stage. 

So, how long are dogs in heat, exactly? And how do you know when your dog is ready to mate? Daily Paws spoke with a veterinarian to find answers.

When Do Dogs Go Into Heat?

This time of fertility and breeding can be a fact of life for your dog at a surprisingly young age. Lonna J. Nielsen, DVM, of Winterset Veterinary Center in Winterset, Iowa, says, "The timing of the first heat varies by the size of the dog. It can be as soon as 6 months of age for small dogs or 1.5 years for giant breeds. Having an intact [non-neutered] male in the house will bring heat in faster for a female. It can be as young as 4 months [old] for small dogs!"

The Female Dog Heat Cycle

how long are dogs in heat infographic showing the four stages
Credit: Kailey Whitman

Just how long are dogs in heat? You might be surprised to find out that your dog's heat cycle can last weeks, and "it is a full 30 days that the female should be confined and not outside alone," Nielsen says.

Stage 1: Signs a Dog Is in Heat

The first signs your dog is in heat are the swelling of her vulva and bright red bloody discharge. This stage is called proestrus. "There will be lots of licking," Nielsen says. A dog in heat can bleed for around seven to 10 days. During this first stage of the reproductive cycle, the female will not accept a male.

Stage 2: When Your Dog Can Become Pregnant

Once the bleeding stops, the second stage of the dog heat cycle, called the estrus stage, has begun. Many people make the mistake of thinking their dog's estrus is done at this point, but this second stage is actually when your dog can become pregnant. "This stage, when the blood is done, is when she will accept a male," Nielsen says. "This lasts an additional seven to 10 days." During this stage, the vulva will become soft and enlarged.

If your dog is in heat and you do not want her to get pregnant, you should be very careful to keep her away from intact males during this time. "Heat, for both males and females, is very intense and instinctive," Nielsen says. "They will breed through fences and kennels, and have been known to break out windows and chew or dig through doors to get together. It is a strong impulse." 

Stage 3: The End of a Dog's Heat Cycle

The last stage of the heat cycle, called diestrus, lasts about two months. It includes reswelling of the vulva and pink discharge, and she will not accept a male. As this final stage ends, the vulva returns to normal and the discharge tapers off. This is the time of pregnancy if your dog mated with a male, though diestrus lasts 60 days whether or not she's carrying puppies.

Stage 4: The Downtime

Anestrus is when your dog isn't in heat. This period of time lasts several months (between three and six months on average, though it varies from dog to dog) before she reenters proestrus.

How Often Do Dogs Go Into Heat?

A dog's heat cycle begins roughly every six months for unspayed female dogs until 8 to 10 years of age. 

Nielsen says in her experience, "They seem to show a preference for spring and fall—when the days lengthen and then when it starts getting cooler.

"The first heat cycle, Nielsen explains, will be light and owners may be thinking This is easy! No big deal. "But when the second cycle comes, it's 'Holy crap!'" she says.

Spaying Your Dog

Having your female dog spayed means she will not experience the estrus cycle or pregnancy at all. If your pup goes into heat before you have her spayed, consult with your veterinarian about the best timing for the surgery. In general, it is best to spay before the dog's first heat ever happens (as early as 8 weeks of age), but the procedure can be done once the first heat has finished.

Spaying has benefits outside of preventing pregnancy, including decreased risks of mammary cancer and pyometra (uterine infection).