Most breeders ask the same question about heat cycles: when is she due? The honest answer is a window, not a date.
The normal gap between heats is 4 to 13 months, about 7 on average. Some bitches stay within days of their own interval from heat to heat; others vary by months.
This guide covers what breed and size predict, how much one bitch can vary, what really shifts her timing, and which changes need a vet.
The short version
- The normal range: 4 to 13 months between heats, about 7 on average.
- Size sets the rough schedule: Small breeds can cycle three times a year and some giant breeds once. Large and giant breeds may not have a first heat until 18–24 months.
- Her own pattern: Some bitches stay within days of their usual gap; others vary by months. Her recorded heats show her range so far.
- Medicines matter: Steroids can hold back a heat, and a person's estrogen gel can make one drag on. Tell your vet about both.
- When to call the vet: No heat by 24 months, heats under 4 months apart, none for 10–12 months, or heat signs past 3 weeks. Pus-like discharge or illness after a heat: the same day, or an emergency vet now if she is very ill.
is the normal gap between heats
About 7 months on average. Some bitches stay within days of their own interval; others vary by months.
Breed and size set the rough schedule
Size gives the rough expectation. Small breeds tend to have their first heat earlier and may cycle three times a year. Large and giant breeds may not start until 18 months to two years, and some giant breeds cycle only once a year.
The reason is growth: a bitch usually reaches puberty 2 to 3 months after she reaches adult size, and small breeds get there first.
| Size | First heat | Gap between heats |
|---|---|---|
| Small and toy breeds | Earlier than average | Some cycle three times a year |
| Most dogs | About 6 months on average; most 6–12 | About twice a year: 4–13 months apart, 7 on average |
| Large and giant breeds | Can be as late as 18–24 months | Some giant breeds cycle only once a year |
VCA Animal Hospitals (Estrus Cycles in Dogs); Threlfall, dvm360 proceedings; Merck Veterinary Manual.
What studies found for named breeds
Only a few breeds have published figures. These are averages from real kennels, with a lot of spread around them.
| Breed | Average gap between heats | Where it comes from |
|---|---|---|
| German Shepherd | 149 days (about 5 months), the shortest of 7 breeds | US study (Sokolowski 1977) |
| German Shepherd | 183 days (about 6 months) in bitches not bred | 798 heats, Kenyan breeders (Mutembei 2000) |
| German Shepherd | 190 days (about 6 months) | 211 heats, Thai kennel (Chatdarong 2007) |
| Labrador Retriever | 252 days (about 8 months), with a wide spread | 68 heats, same Thai kennel (Chatdarong 2007) |
| Basenji | Once a year, usually September–October north of the equator | Basenji Club of America; vet reviews |
Sokolowski JH et al., JAVMA 1977; Mutembei HM et al., J S Afr Vet Assoc 2000; Chatdarong K et al., J Small Anim Pract 2007; Basenji Club of America; Johnson and Threlfall, dvm360 proceedings.
German Shepherds run short. They had the shortest gap of the seven breeds in a US study, and a theriogenology review notes that 4 to 4.5 months can be normal and fertile in the breed. They can start late, too: Kenyan German Shepherds averaged about 17 months at their first heat.
The Thai Labradors averaged about 8 months, but the spread was wide: a standard deviation of 114 days, almost 4 months, so many were well under or over that.
Once-a-year cycling is normal for the Basenji, the Tibetan Mastiff and the Dingo. Basenjis north of the equator mostly come into heat in September or October.
We found no published figures for Chihuahuas, French Bulldogs, Golden Retrievers or Great Danes. For those breeds, use the size table above and your own records.
How much one bitch's timing can vary
In a study of 126 gaps between heats, the variation from heat to heat within the same bitch was about twice the variation between the bitches' own averages.
The authors concluded that the gap could not predict the next heat for a single bitch, although some bitches were very consistent. Concannon's review puts it the same way: within one bitch, intervals range from highly variable to regular, perhaps within 5–10 days of a 7-month cycle.
That is why keeping records across several heats matters. They show her range so far, and that range can shift as you add more.
Example: one bitch's own range
An illustration, not a study dog: a Labrador whose first four heats came at these points.
- First heat: age 11 months
- Second heat: 7.5 months later
- Third heat: 6 months later
- Fourth heat: 8 months later
Her own gap runs 6 to 8 months, so the 8-month gap before the fourth heat isn't late for her. Plan the fifth as a window from about 6 to 8 months after the fourth, and start watching at the early end.
What shifts the timing: age, medicines, illness and housing
Between heats a bitch rests in anestrus, which lasts 8 to 40 weeks. What ends it is poorly understood: pheromones, possibly daylight, and an internal yearly rhythm all seem to play a part.
Age: irregular at both ends of life
Young bitches' cycles are often irregular, and it can take up to two years for them to become regular. The first heat can also be long or split.
Irregular cycles are typical in older dogs too. An unspayed bitch keeps coming into heat for life, but in older bitches heats are often less noticeable, and an elderly bitch can still conceive.
Fertility may decline from about 6 years, with smaller litters. See our guide to deciding when to retire a breeding dog.
Medicines: steroids and hormone creams
Steroid medicines can hold back a heat. In a 2026 study, 17 of 28 bitches on prednisolone had a long pause between heats, and 8 of them cycled again once the dose was cut or stopped. Two started puberty late.
VCA lists exposure to oral and topical steroids, including those used by people, among the causes of a bitch not cycling. Estrogen works the other way: a person's hormone-replacement gel or spray can cause a heat that goes on too long.
Tell your vet about every medicine she gets, and about any steroid or hormone cream or gel used in the house. Don't stop a prescribed medicine on your own: ask your vet.
Illness: Cushing's, infections, and the thyroid question
Infections and illnesses such as kidney failure and Cushing's disease can stop normal heat cycles. In the same 2026 study, 2 of 6 bitches with Cushing's had a long pause between heats.
Hypothyroidism is often blamed but rarely confirmed. Merck notes that hypothyroid bitches may have longer gaps between heats. But in two controlled studies, bitches made hypothyroid had the same gap as healthy ones, though their puppies were smaller and more died around birth.
A theriogenology review calls thyroid disease "commonly reported but rarely confirmed" as a cause, and none of the 8 hypothyroid bitches in the 2026 study had a changed cycle. A thyroid panel is still part of the usual workup, with general blood and urine tests.
Body condition and workload
VCA lists malnutrition and excessive physical activity among the reasons a young bitch may not have started cycling by two years of age.
Other bitches: earlier first heats, no proven syncing
A young bitch housed with mature cycling bitches tends to reach her first heat sooner. VCA lists a lack of contact with other cycling bitches among the reasons a young bitch may not have started cycling.
Pheromones are thought to be one of the cues that end the rest between heats, but we found no study showing that bitches living together truly synchronise. Track each bitch on her own.
Season: little effect, except the Basenji
Domestic dogs are not seasonal breeders. A study of seven breeds found heats in every season with no real difference, and a UK guide-dog colony found no seasonal pattern across 3,624 heats.
One vet review reports a small rise in late winter and early spring, and a kennel in Thailand saw fewer heats in the hot summer. Don't plan around the calendar. The exception is the Basenji, which usually cycles once a year in the autumn.
When a change in timing needs a vet
Most variation is normal. These patterns are not:
| Pattern | What it can mean | What to do |
|---|---|---|
| Never in heat by 24 months | A late but normal start, a silent heat you missed, a developmental problem, or illness | Vet exam. Progesterone above 2 ng/mL shows a heat in the last 2–3 months; a lower result can't rule out an earlier one |
| Heats under 4 months apart, again and again | Often linked to failed matings; the cause is usually not found. Rule out a split heat. 4–4.5 months can be normal in German Shepherds | Vet; progesterone through the next heat to check she ovulates |
| No heat for more than 10–12 months (not a once-a-year breed) | Steroid medicines, Cushing's, other illness, or age. Thyroid disease is often blamed but rarely the cause | Vet workup: every medicine she has had, and blood tests including a thyroid panel |
| Heat signs lasting more than 3 weeks | Longer than usual. Abnormal by 35–40 days: usually ovarian cysts, or estrogen such as a person's HRT gel | Call your vet |
| Discharge between heats, or a clear change from her own pattern | Many causes, from vaginal inflammation to uterine disease. Worth checking even inside the normal range | Call your vet and bring your records |
| Pus-like or foul-smelling discharge, or she is unwell and drinking more, after a heat | Pyometra, a life-threatening uterine infection: usually 4–6 weeks after a heat (2–16). With a closed pyometra there may be no discharge | Vet the same day; an emergency vet now if she is very ill, vomiting or has a swollen belly |
Johnson and Threlfall, dvm360 proceedings; Merck Veterinary Manual; VCA Animal Hospitals; Bauer et al., Theriogenology 2026; Panciera et al. 2007 and 2012.
The thresholds come from theriogenology reviews: no heat by 24 months, gaps under 4 months, and more than 10–12 months without a heat are all reasons to refer. Each stage of heat averages about 9 days, and a season is not considered abnormally long until 35 to 40 days. Vaginal inflammation can affect a bitch at any age.
Build her profile from her own heats
The most useful thing you can do is build a profile for each breeding bitch from her own heats, not breed averages. After several recorded heats you'll have a first picture of:
- Her usual gap (for example, "7–8 months, never under 6")
- How long her proestrus usually lasts (about 9 days is average)
- How long her standing heat lasts, and her signs
- Any split or silent heats, and anything that came before a change, such as a new medicine or an illness
If her gaps still swing by months after several heats, that is her pattern. Plan a wide window and start progesterone testing on her signs, not the calendar.
Use the Heat Cycle Calculator to estimate her next heat from her own average interval, and the Breeding Window Calculator once a heat starts.
For the four stages, split heats and silent heats in detail, see our guide to understanding the dog heat cycle.
Heat cycle timing FAQs
1How accurate is the 'every 6 months' rule?
It's a rough average, not a rule. The normal gap between heats is 4 to 13 months, about 7 on average. Some small-breed bitches cycle three times a year, some giant breeds only once, and Basenjis once a year. Within one bitch, some stay within days of their usual gap while others vary by months. After three or more recorded heats, her own range tells you more than any average.
2My dog's heat is late — should I be worried?
Look at her first, then the calendar. If she seems unwell, or has any discharge, call your vet whatever the date. If she is well, compare the gap with her own past heats, since one bitch's gaps can vary a lot, and check whether she is on a steroid medicine or has been near a steroid cream: steroids can hold back a heat. See your vet if it has been more than 10–12 months since her last heat (unless she is a once-a-year breed such as the Basenji), if the gap is far outside her own pattern, or if she has never had a heat by 24 months; the wider 4–13 month range for dogs in general doesn't override those lines. A progesterone blood test above 2 ng/mL (6.4 nmol/L) shows she has cycled in the last two to three months, though a lower result can't rule out an earlier heat you missed; your vet will interpret it.
3Can two dogs living together sync their heat cycles?
There's no good evidence that they do. Living with cycling bitches can matter in other ways: a young bitch housed with mature cycling bitches tends to reach her first heat sooner, and pheromones are thought to be one of the cues that end the rest between heats. Track each bitch on her own, and never assume that one coming into season means another is fertile.
4Does season affect when dogs come into heat?
Very little for most dogs. Domestic dogs are not seasonal breeders: a study of seven breeds found heats in every season with no real difference, and a UK guide-dog colony found no seasonal pattern across 3,624 heats. One vet review reports a small rise in late winter and early spring, and a kennel in Thailand saw fewer heats in the hot summer. The clear exception is the Basenji, which usually cycles once a year, in September or October north of the equator.
5What is a split heat and is it a problem?
In a split heat, proestrus starts normally, with swelling and discharge, then stops before she reaches standing heat. Two to four weeks later it starts again and usually runs through a normal, fertile heat. Split heats are most common in young bitches around their first heat, rarely happen again and again in the same bitch, and don't cause infertility, though the false start makes breeding timing harder. Record both starts, and talk to your vet if it keeps happening.
6At what age does a dog have her first heat, and when do cycles become regular?
Around 6 months on average, with most between 6 and 12 months and a full range of about 4 to 24 months. Puberty usually comes 2 to 3 months after a bitch reaches adult size, so small breeds tend to start earlier, while large and giant breeds may not start until 18 to 24 months. Early cycles are often irregular, and it can take up to two years for a bitch to settle into regular cycles, so don't judge her normal pattern from her first heat.
7What is a silent heat, and how would I know?
In a silent heat, a bitch ovulates but the usual signs, vulvar swelling and discharge, aren't seen, so the heat is easy to miss. Some vets think an unnoticed first heat is common, which can make a young bitch look as if she hasn't started cycling. A progesterone blood test above 2 ng/mL (6.4 nmol/L) shows she has cycled in the last two to three months, though a lower result can't rule out an earlier heat; vaginal cytology every one to two weeks can catch the next one.
8Can a health problem change my dog's cycle timing?
Yes. Steroid medicines have the clearest evidence: in one study, 17 of 28 bitches on prednisolone had a long pause between heats, and 8 cycled again once the dose was cut or stopped. Cushing's disease, kidney disease, infections and ovarian cysts or tumours can also upset the cycle. Hypothyroidism is often blamed but rarely confirmed: in controlled studies it did not change the gap between heats. A vet workup usually includes blood tests with a thyroid panel and a review of every medicine she has had. Bring your records: the pattern over several heats tells your vet more than one late season.
9Do heat cycles change as a dog gets older?
An unspayed bitch keeps coming into heat for life, since dogs don't have a menopause, but in older bitches heats are often less noticeable and cycles less regular. A sudden change in an older bitch is different: discharge that looks like pus or smells bad, or a bitch who is unwell and drinking more in the weeks after a heat, needs a vet the same day to rule out pyometra, a life-threatening uterine infection that is most common in older unspayed bitches. If she is very ill, vomiting or has a swollen belly, go to an emergency vet now: with a closed pyometra there may be no discharge at all.
Related Tools
References & sources
- Merck Veterinary Manual. Breeding Management of Bitches. — The gap between heats is normally 4–13 months, 7 on average; proestrus and estrus each last 3 days to 3 weeks, 9 days on average.
- Merck Veterinary Manual. Infertility in Dogs and Cats. — Hypothyroid bitches may have longer gaps between heats; prolonged heat can come from estrogen-producing ovarian cysts, ovarian tumours or estrogen from outside the body, including human hormone-replacement therapy.
- Merck Veterinary Manual (owner edition). Reproductive Disorders of Female Dogs. — Follicular cysts should be suspected after more than 40 days of heat signs; pyometra mainly affects dogs over 5 and tends to follow heat by 4–6 weeks.
- Concannon PW. Reproductive cycles of the domestic bitch. Animal Reproduction Science 2011;124:200–210. — Dogs are typically non-seasonal; gaps of 5–12 months, commonly 6–7, from highly variable to regular within a bitch; anestrus of 8–40 weeks ended by pheromones, possibly daylight, and an internal yearly rhythm.
- Bouchard G, et al. Seasonality and variability of the interestrous interval in the bitch. Theriogenology 1991;36:41–50. — Variation within a bitch about twice the variation between bitches' averages; the gap could not predict the next heat for a single bitch, though some were very consistent; no seasonal pattern within individual years.
- Sokolowski JH, et al. Seasonal incidence of estrus and interestrous interval for bitches of seven breeds. JAVMA 1977;171:271–273. — Heats in all seasons with no significant seasonal difference; German Shepherds had the shortest gap, 149 days.
- Mutembei HM, et al. A retrospective study on some reproductive parameters of German shepherd bitches in Kenya. J S Afr Vet Assoc 2000;71:115–117. — Mean age at first heat 519 days; gap 183 days in bitches not bred.
- Chatdarong K, et al. Seasonal and breed effects on reproductive parameters in bitches in the tropics. J Small Anim Pract 2007;48:444–448. — Labrador gap 252 days (SD 114) against German Shepherd 190 days (SD 61); fewer heats in summer.
- Wigham EE, et al. Seasonality in oestrus and litter size in an assistance dog breeding colony in the United Kingdom. Veterinary Record 2017;181:371. — 3,624 heats in 568 bitches; no regular seasonal pattern.
- Panciera DL, et al. Effect of short-term hypothyroidism on reproduction in the bitch. Theriogenology 2007;68:316–321. — No difference in the gap between heats; longer labour, lower puppy birth weight and survival.
- Panciera DL, et al. Reproductive effects of prolonged experimentally induced hypothyroidism in bitches. J Vet Intern Med 2012;26:326–333. — No difference in the gap between heats; higher puppy mortality and lower birth weight, reversed by treatment.
- Bauer E, et al. Influence of glucocorticoid therapy, Cushing's syndrome and hypothyroidism on the reproductive cycle and AMH levels in the bitch. Theriogenology 2026;268:118261. — Prolonged anestrus in 17 of 28 bitches on prednisolone (8 returned to heat when the dose was cut or stopped) and 2 of 6 with Cushing's; no cycle changes in 8 hypothyroid bitches.
- Johnson C. Approach to infertility (Proceedings). dvm360, 2008. — Never cycled by 24 months; progesterone to detect a missed heat; thyroid "commonly reported but rarely confirmed"; steroids and prolonged anestrus; the usual workup; gaps under 4 months (4–4.5 normal in some German Shepherds) and over 12 months (normal in Basenji, Tibetan Mastiff, Dingo); split heats; a season abnormal at 35–40 days.
- Threlfall W. Canine estrous cycle and ovulation (Proceedings). dvm360, 2009. — Puberty 2–3 months after adult size, 6–12 months on average (range 4–24); a first heat can be long or split and comes sooner when housed with cycling bitches; Basenjis cycle about once a year; a small late-winter and early-spring rise.
- Threlfall W. Canine reproductive diseases and abnormalities (Proceedings). dvm360, 2008. — Refer at no heat by 24 months or more than 10 months without one in an adult bitch; possible fertility decline after 6 years.
- VCA Animal Hospitals. Estrus Cycles in Dogs. — First heat around 6 months on average, 18–24 months in some large and giant breeds; small breeds up to three heats a year, giant breeds sometimes one; up to two years for cycles to become regular.
- VCA Animal Hospitals. Infertility in Female Dogs. — Never cycled by two years and its causes, including malnutrition, heavy exercise, medications and no contact with cycling bitches; irregular cycles in young and older dogs; illness and oral or topical steroids, including human ones; silent heat.
- VCA Animal Hospitals. Pyometra in Dogs. — Life-threatening; most common in older dogs; usually 4–6 weeks after heat but 2–16; closed pyometra makes dogs severely ill very quickly.
- VCA Animal Hospitals. Vaginitis in Dogs. — Can affect any female dog at any age.
- Johannes JE. The Basenji Annual Estrus: African Origins. Basenji Club of America, 2002. — Basenji heats mostly in September and October in the Northern Hemisphere.
- Eldredge DM. Dogs Do Not Go Through Menopause. Whole Dog Journal, 2025. — Unspayed dogs keep coming into heat; heats are less noticeable in older dogs; an elderly bitch may still become pregnant.
This article is for educational purposes and does not replace professional veterinary care. If your bitch seems unwell after a heat, contact a veterinarian straight away.