How long are dogs in heat?
A heat usually lasts about 2 to 3 weeks. It has two visible stages. Proestrus comes first: the vulva swells, there's a bloody discharge and males get interested, but she won't accept them yet. It lasts about 9 days on average (2 days to 3 weeks, says Merck). Estrus follows, when she's willing to mate and can get pregnant. It also averages about 9 days, but can be as short as 3 days or as long as 3 weeks.
How often do dogs go into heat?
Most dogs come into heat about twice a year. The Merck Veterinary Manual gives an average gap of 7 months, with a normal range of 4 to 13 months. Large breeds such as Great Danes may cycle every 9 to 12 months, and Basenjis only once a year. A dog's own gap is usually fairly steady, so if you know her history, enter that.
Proestrus ≈ days 1–9 · Estrus (fertile) ≈ days 10–18
Worked example
Her last heat started January 15, 2026, and she usually cycles every 7 months. Next heat: August 15, 2026. Proestrus would run about August 15–23 and estrus about August 24 to September 1. The one after that would start around March 15, 2027.
The four stages of the cycle
| Stage | Typical length | What you'll notice |
|---|---|---|
| Proestrus | About 9 days (2 days–3 weeks) | Swelling, bloody discharge, males interested |
| Estrus | About 9 days (3 days–3 weeks) | Willing to mate; discharge lighter and pinkish |
| Diestrus | About 2 months | Signs fade; false pregnancy signs are possible |
| Anestrus | About 4–6 months | Rest before the next cycle |
When to call your vet
- A heat that seems much longer or heavier than usual, or no heat for well over a year in an unspayed adult dog.
- Drinking or peeing more, low energy or a poor appetite in the weeks or months after a heat. The AKC warns these can be signs of pyometra, a serious uterine infection that can occur 4 weeks to 4 months after a heat, especially in older unspayed dogs.
- Any questions about breeding timing. Progesterone blood tests are far more precise than a calendar.
Limits
- First heats often come at 6 to 16 months depending on size and can be irregular or "split."
- Small dogs can have quiet heats you might miss, which throws off the count.
- This is a calendar estimate. It can't tell you when she's ovulating.
Sources
- Merck Veterinary Manual (pet owner version): Management of Reproduction in Dogs interval 4–13 months (average 7); proestrus and estrus lengths.
- AKC: How Long Are Dogs in Heat? stages, signs, pyometra.
- AKC: How Often Do Dogs Go Into Heat? age at first heat.