Dog in Heat: Stages, Signs, How Long It Lasts & What to Do (Vet Guide)

Vet-reviewed by Veterinary Reviewer, DVM · Last reviewed June 2026

A dog "in heat" is in the fertile phase of her reproductive cycle. Most unspayed dogs come into heat about twice a year, starting between 6 and 12 months of age (later in large breeds). A heat lasts 2–4 weeks, with telltale signs including a swollen vulva, bloody discharge, and increased attention from male dogs.

What does “in heat” mean?

Heat (estrus) is part of the estrous cycle — the recurring period when an intact (unspayed) female dog is fertile and can become pregnant. Spaying removes the ovaries and uterus, so spayed dogs do not go into heat.

What are the 4 stages of the dog heat cycle?

Stage	Typical length	What happens
Proestrus	~9 days	Swollen vulva, bloody discharge; attracts males but will not mate
Estrus	~9 days (range 3–21)	The fertile window; discharge lightens; she becomes receptive to males
Diestrus	~2 months	Cycle winds down; pregnant or not, hormones shift
Anestrus	~4 months	Resting phase until the next cycle

What are the signs a dog is in heat?

Common signs include a swollen vulva, blood-tinged to straw-colored discharge, more frequent urination (scent-marking), restlessness or clinginess, “flagging” (moving the tail to one side), and strong interest from male dogs. Some dogs keep themselves very clean, so discharge is not always obvious.

How long does a dog stay in heat?

The visible heat usually runs 2 to 4 weeks. Cycle frequency varies by size: small breeds may cycle every 4–6 months, while some large and giant breeds cycle only once a year.

How do I care for a dog in heat?

Keep her securely away from intact males at all times — dogs can mate through fences and escape even determined barriers. Walk her on a leash, supervise the yard, and consider a washable doggy diaper for discharge. Provide extra comfort and patience with behavior changes.

If you do not plan to breed her, ask your vet about spaying, which also lowers the risk of mammary tumors and pyometra — a life-threatening uterine infection.

Related guides: Dog pooping blood · Hot spots on dogs

Frequently asked questions

At what age does a dog first go into heat?

Usually 6–12 months. Large and giant breeds may not start until 18–24 months of age.

Can a dog get pregnant on her first heat?

Yes — though many vets advise against breeding during the first heat cycle.

When can I spay my dog after a heat?

Many vets recommend waiting a few weeks after the heat ends. Ask your vet for the best timing for your dog.