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An unspayed female dog who is ovulating and ready to breed is commonly referred to as being “in heat.” Here’s what you should know about the dog heat cycle. 

By Lacey Howard
Updated March 24, 2021
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Ryan Murphy / Getty
| Credit: Ryan Murphy / Getty

If your sweet little girl is not yet spayed, don't be caught off guard if she gets her period. You should be in the know about the female canine heat cycle, called estrus—including the signs of a dog in heat and how long a dog stays in heat.

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 four months for small dogs!"

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. "There will be lots of licking," Nielsen says. A dog in heat can bleed for around 7 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 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 7 to 10 days." During this stage, the vulva will become soft and enlarged.

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

The final 7 to 10 days is the last stage of the heat cycle. It includes reswelling of the vulva and pink discharge, but she will not accept a male. As this final stage ends, the vulva returns to normal and the discharge tapers off.

So just how long does a dog stay in heat? You might be surprised to find out that your dog's heat cycle might be for an entire month! "It is a full 30 days that the female should be confined and not outside alone," Nielsen says.

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." 

How Often Do Dogs Go Into Heat?

A dog's heat cycle recurs roughly every 6 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!" Nielsen says.

Spaying Your Dog

Having your female dog spayed means she will not experience estrus 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 eight weeks of age), but the procedure can be done once the first heat has finished.