The dog heat cycle: how often, how long and the signs

How often dogs come into heat

The time from the start of one heat to the start of the next is called the interestrous interval. Merck’s professional chapter puts it at 4 to 13 months in normal dogs, with 7 months the average. Merck’s page for owners adds that many dogs cycle about twice a year, and that some large breeds, such as Great Danes, may normally cycle every 9–12 months.

Other references give slightly different numbers. A veterinary conference paper by Threlfall puts the average at about six months, with a range of 4 to 24, and notes that basenjis — like dingoes and wolves — usually come into heat only once a year. The same paper says puberty usually comes 2–3 months after a dog reaches adult body size: on average at 6 to 12 months of age, sooner in small breeds than in large ones.

Threlfall adds two details owners notice. Heats can start in any month, with a small but real increase in late winter and early spring. And a young dog’s first heat can be unusual — long, or split into two parts — and tends to come sooner if she lives with older females that are cycling.

The practical point: a dog’s own past intervals are a better guide than any average. If you’ve written down the start of her last two heats, that gap is the first thing to use. Merck’s professional chapter adds a caution about those notes: even an observant owner may not spot the true start of proestrus for a few days, so the date you wrote down may be a little late.

The four stages and what you’ll see

The cycle has four phases. The numbers are averages with wide ranges, and Merck stresses that normal dogs can differ from the average.

Threlfall’s paper gives similar averages, 9 days each, with ranges of 3–17 days for proestrus and 3–18 days for estrus, and suggests that proestrus and estrus together normally don’t run past 21 days. Merck notes that ovulation happens during estrus, about 2–3 days after the hormone peak that triggers it, and that dogs ovulate whether or not they are bred.

The fertile days come from a blood test, not the calendar

Merck is clear that estrus behavior doesn’t line up precisely with the fertile period. Standing for the male may begin before or after the hormone peak, and its length varies. Vets therefore time breedings with blood tests.

The usual method is progesterone testing every other day once a vaginal smear shows she is well into her heat. The day progesterone first rises clearly above baseline — usually to about 1.5–2.5 ng/mL — matches the LH surge, the spike of luteinizing hormone that triggers ovulation, and becomes day 0. Merck’s chapter advises natural breeding or fresh-semen insemination between days 3 and 6 after that rise, and a recheck 2–3 days later to confirm she has ovulated.

Two facts explain why calendar counting fails. Merck says fresh semen can live in her reproductive tract for 9 or more days, so a breeding well before the fertile days can still lead to puppies. And her eggs need 2–3 days to mature after ovulation, so the fertile days come later than the first day she stands. If you plan to breed her, Merck’s owner page suggests talking to your vet before she comes into season and calling as soon as you notice the first signs.

A worked example: her next heat

Say her last heat started on Sunday, March 1, 2026. Day 0 is March 1. Our calculator, using Merck’s figures, gives:

1. Typical next heat: 7 months later, Thursday, October 1, 2026.
2. Normal window: 4 to 13 months later, Wednesday, July 1, 2026 to Thursday, April 1, 2027.
3. This heat’s stages, if she follows the averages: proestrus about March 1–9, estrus about March 10–18. Either stage could be as short as 3 days or as long as 3 weeks.

Now add her own history. If the heat before started on Wednesday, August 20, 2025, her last interval was 193 days. If that interval repeats, her next heat would start around Thursday, September 10, 2026 — three weeks earlier than the average date.

A calculator that simply adds six months would say September 1, 2026. That is inside the normal window, but it presents one date as if it were known, when the published range is nine months wide and her own pattern counts for more than either average.

When to talk to your vet

Merck notes that unusual cycles need a vet to tell normal variation from a real problem. Talk to your vet if:

• bleeding and standing heat together go on for more than about 3 weeks;
• she hasn’t come into heat by the end of the normal window, or her cycles suddenly change pattern;
• you want to breed her: book progesterone timing before or at the first sign of heat;
• you don’t plan to breed her and want to discuss spaying or other ways to prevent a pregnancy.

This is general information, not veterinary advice — talk to your vet.

Next step

Enter the first day of her last heat, and the one before it if you have it, in our dog heat cycle calculator to see the typical date, the normal window and the date her own interval points to. If she has been bred, switch to the dog pregnancy calculator and read our dog pregnancy timeline.