The canine estrous cycle — commonly called the heat cycle — is the reproductive cycle that determines when a female dog is fertile and able to conceive. Unlike humans, who have a monthly menstrual cycle, dogs cycle less frequently and follow a pattern with four distinct stages, each driven by specific hormonal changes.
Understanding these stages is essential for responsible breeding. Knowing what is happening hormonally and physically at each phase allows breeders to identify the optimal breeding window, recognize abnormalities early, and make informed decisions about when — and when not — to breed.
make up the complete canine estrous cycle
Proestrus → Estrus → Diestrus → Anestrus, then the cycle repeats
The four stages of the estrous cycle
Every heat cycle consists of four stages that occur in the same order. The entire cycle — from the start of one heat to the start of the next — typically spans 6 to 12 months depending on the individual dog and her breed.
| Stage | Duration | Key Signs | Hormone Activity |
|---|---|---|---|
| Proestrus | 7–10 days (range: 3–17) | Bloody vaginal discharge, swollen vulva, attracts males but refuses mating, increased urination | Rising estrogen, follicles developing on ovaries |
| Estrus | 5–9 days (range: 3–21) | Discharge lightens to straw/pink, vulva softens, female "flags" (stands and deflects tail), accepts male | Estrogen peaks then drops, LH surge triggers ovulation, progesterone rises sharply |
| Diestrus | 60–90 days | Discharge stops, vulva returns to normal size, female rejects males, behavioral return to normal | Progesterone dominant (whether pregnant or not), gradually declines after ~day 60 |
| Anestrus | 4–12 months (variable) | No reproductive activity, no discharge, no interest from males, uterus recovers and repairs | Hormonal quiescence — the reproductive system rests and prepares for the next cycle |
Durations from Concannon PW, Clinical Theriogenology; Feldman & Nelson, Canine and Feline Endocrinology. Individual variation is normal.
Stage 1: Proestrus — the opening act
Proestrus is the first visible sign that a heat cycle has begun. It typically lasts 7 to 10 days, though it can range from 3 to 17 days in some individuals.
What you will see
- Vulvar swelling — The vulva becomes noticeably enlarged, sometimes 2-3 times its normal size. This is one of the earliest and most reliable visual indicators
- Bloody vaginal discharge — Ranges from light spotting to heavy dripping depending on the dog. Some dogs are meticulous self-cleaners and you may only notice spots on bedding
- Behavioral changes — Increased restlessness, more frequent urination (marking to advertise her status), may become clingy or unusually affectionate
- Attracting males — Male dogs can detect pheromones in her urine from considerable distances. However, she will reject mating attempts during proestrus — snapping, sitting down, or walking away
What is happening hormonally
Estrogen levels are rising steadily as follicles develop on the ovaries. This estrogen drives the vulvar swelling, the bloody discharge (caused by the increased blood flow to the uterine lining), and the pheromone production that attracts males. The key to watch: the discharge will change color as estrogen peaks and gives way to the next stage.
Stage 2: Estrus — the fertile window
Estrus is the only stage during which the female is fertile and will accept mating. It typically lasts 5 to 9 days, but can range from 3 to 21 days. This is the critical window for breeding.
What you will see
- Discharge changes color — From dark red/bloody to lighter straw, pink, or clear. Some breeders describe this as the discharge becoming "watery." This color shift is a key indicator of the transition from proestrus to estrus
- Vulva softens — While still enlarged, the vulva becomes noticeably softer and more pliable compared to the firm swelling of proestrus
- Standing heat/flagging — When pressure is applied to the lower back or hindquarters, the female stands firmly and deflects her tail to the side. This is called "flagging" and is the classic behavioral sign of receptivity
- Active solicitation — The female may actively seek out males, present her hindquarters, and play bow
What is happening hormonally
This stage involves the most dramatic hormonal shifts of the entire cycle. Estrogen peaks and then drops rapidly. The pituitary gland releases a luteinizing hormone (LH) surge that triggers ovulation — typically occurring about 2 days after the LH surge. Progesterone begins rising sharply after the LH surge.
This is why progesterone testing is the gold standard for breeding timing. Ovulation occurs when progesterone reaches approximately 5 ng/mL, and eggs require an additional 48 hours to mature before they are fertile. The optimal breeding window is typically 4 to 6 days after the LH surge.
Stage 3: Diestrus — the luteal phase
Diestrus begins when the female stops accepting the male and lasts approximately 60 to 90 days. This stage occurs whether or not the dog is pregnant — the hormonal pattern is remarkably similar in pregnant and non-pregnant dogs.
What you will see
- Discharge stops — Any remaining discharge clears up within the first few days
- Vulva returns to normal — Swelling gradually resolves over 1-2 weeks
- Behavioral normalization — The female returns to her pre-heat temperament and no longer attracts or accepts males
- Possible false pregnancy — Because progesterone levels rise regardless of pregnancy status, some dogs develop false pregnancy symptoms: nesting behavior, mammary gland development, even milk production. This is hormonally normal, though it can be confusing
What is happening hormonally
Progesterone is the dominant hormone during diestrus, secreted by the corpora lutea on the ovaries. In a pregnant dog, progesterone sustains the pregnancy. In a non-pregnant dog, progesterone follows a nearly identical rise-and-fall pattern, declining naturally after about 60 days. This decline triggers the end of diestrus and the transition to anestrus.
Stage 4: Anestrus — the resting phase
Anestrus is the reproductive "off season." There is no visible reproductive activity — no discharge, no swelling, no behavioral changes. This stage is highly variable in length, lasting anywhere from 4 to 12 months depending on the individual and her breed.
During anestrus, the uterus is recovering and repairing from the previous cycle. The endometrial lining regenerates, and the reproductive system prepares for the next cycle. The dog's body essentially decides when it is ready to cycle again based on internal hormonal signals.
The length of anestrus is the primary reason why cycle intervals vary so much between dogs and even between cycles in the same dog. Factors like breed size, age, nutrition, stress, and photoperiod all influence how long anestrus lasts.
How cycle timing varies by breed size
One of the most significant factors affecting heat cycle frequency is breed size. Smaller breeds tend to cycle more frequently, while larger breeds cycle less often. This is a general pattern with considerable individual variation.
| Breed Size | First Heat Age | Typical Cycle Interval | Example Breeds |
|---|---|---|---|
| Toy (under 10 lb) | 5–8 months | Every 4–6 months | Chihuahua, Yorkie, Maltese, Pomeranian |
| Small (10–25 lb) | 6–10 months | Every 5–7 months | Shih Tzu, Miniature Dachshund, Cavalier |
| Medium (25–50 lb) | 8–12 months | Every 6–8 months | Beagle, Cocker Spaniel, English Bulldog |
| Large (50–90 lb) | 10–18 months | Every 6–9 months | Labrador, Golden Retriever, German Shepherd |
| Giant (90+ lb) | 12–24 months | Every 8–12 months | Great Dane, Saint Bernard, Mastiff |
| Basenji (unique) | 10–14 months | Once per year (seasonal) | The only breed that cycles annually, typically in autumn |
General ranges from veterinary literature. Individual dogs vary — track your own dog's pattern over 3+ cycles for the most reliable prediction.
These ranges explain why a Chihuahua breeder expecting cycles "every 6 months" may see cycles as frequently as every 4 months, while a Great Dane breeder might wait 10-12 months between heats — both perfectly normal for their breed size.
Recognizing abnormal cycles
While natural variation is normal, some cycle patterns warrant veterinary evaluation:
- No heat by 24 months — While giant breeds may have their first heat as late as 18-24 months, no heat cycle at all by age 2 should be investigated
- Cycles closer than every 4 months — This can indicate ovarian cysts or other hormonal imbalances
- Cycles farther apart than every 12 months — In breeds that normally cycle every 6-8 months, significant lengthening may indicate hypothyroidism or other endocrine issues
- Prolonged bleeding beyond 21 days — Persistent bloody discharge may indicate a uterine infection, clotting disorder, or other pathology
- Foul-smelling discharge at any time — This is a red flag for pyometra (uterine infection), especially in unspayed females over 6 years old
Practical implications for breeders
Understanding the estrous cycle has direct implications for breeding management:
- Breeding timing — The fertile window during estrus is narrow. Progesterone testing identifies it precisely, removing guesswork from the equation
- Health testing completion — All required health testing should be completed before the planned breeding heat. OFA evaluations, DNA panels, and specialist exams take time to schedule and process
- Age considerations — Most breed clubs recommend breeding no earlier than the second or third heat, and no later than age 5-7 depending on breed. See our guide on when a dog is too old to breed for detailed breed-specific recommendations
- Record keeping — Tracking cycle dates, durations, and observations over multiple cycles builds a picture of each dog's individual pattern. This data is invaluable for planning. See our guide on why record-keeping matters for what to track and how
- Timing variation — Understanding that cycle intervals naturally vary prevents unnecessary panic when a heat arrives a few weeks early or late
Split heats and silent heats
Split heat
A split heat occurs when a dog begins showing proestrus signs (swelling, discharge) that then stop, only to restart days or weeks later. The first episode represents an incomplete hormonal surge — the follicles began developing but didn't reach ovulation. The second episode typically progresses through all stages normally.
Split heats are most common in young dogs during their first few cycles as the hormonal system matures. They are generally not a cause for concern but should be noted in breeding records.
Silent heat
In a silent heat, the hormonal cycle occurs normally but the external signs — discharge, vulvar swelling, behavioral changes — are absent or so subtle they go unnoticed. The dog ovulates and is fertile despite showing no outward indication.
Silent heats are more common in young dogs and certain individuals. They pose a challenge for breeders who rely on visual signs for timing, which is another reason progesterone testing is the most reliable breeding tool available.
Dog heat cycle FAQs
How long does a dog stay in heat?
How often do dogs go into heat?
What is a silent heat?
Can a dog get pregnant on her first heat?
How do I know when my dog is ready to breed?
Do dogs go through menopause?
Related Tools
Sources: Concannon PW. Reproductive cycles of the domestic bitch. Animal Reproduction Science (2011) 124:200–210; Feldman EC, Nelson RW. Canine and Feline Endocrinology (4th ed.). Saunders; Linde-Forsberg C. Canine reproduction. BSAVA Manual of Canine and Feline Reproduction and Neonatology; Johnston SD, Root Kustritz MV, Olson PNS. Canine and Feline Theriogenology. Saunders; Merck Veterinary Manual (Estrous Cycle and Mating chapters). This article is for educational purposes and does not replace professional veterinary care.