At what age do dogs go into heat?

As someone who works with dogs and their caregivers, I find that knowing when a female dog will go into heat changes many practical choices: timing a spay, planning or delaying a litter, avoiding accidental mating, and preparing for behavior and hygiene shifts. This knowledge often reduces stress for owners and improves outcomes for dogs. I’ll walk through the ages and biology you’re likely to see, practical steps to take, and what should prompt a call to your veterinarian.

Why every dog owner should understand heat cycles

Owners need to know heat timing for several everyday reasons. If you’re considering breeding, the age of first heat helps you plan the first breeding cycle and arrange health screenings. If you plan to spay, timing the operation relative to a heat cycle can affect recovery and complication risk, so many vets will discuss optimal windows. Even if breeding isn’t on the table, intact males in the neighborhood or at dog parks can lead to frantic chases and accidental matings, which are emotionally and financially stressful.

In homes with multiple dogs, a female in heat will draw attention from intact males and sometimes trigger competition between household dogs. I typically hear from owners who suddenly face escape attempts or marking behavior they hadn’t seen before. These scenarios also create practical needs—secure exits, different walking routines, and extra cleaning. If your dog shows unusual signs—very heavy bleeding, lethargy, fever, or severe behavioral change—seek veterinary advice promptly, because some problems during or after heat can be medical emergencies.

Typical timing: when most dogs experience their first heat

Most small-breed dogs have their first heat around 6–12 months of age, and many medium breeds fall in a similar window. Larger and giant breeds tend to mature more slowly; first heat often occurs around 9–18 months. These are averages, so individual dogs can be earlier or later. Some toy breeds may cycle before six months, while some giant breeds may not show a first heat until after 18 months.

Once a dog starts cycling, many bitches come into heat roughly twice a year, so every six to eight months, though one cycle per year is seen in some dogs and breeds that are seasonally influenced. A heat episode typically lasts about 2–4 weeks from the start of proestrus through the end of estrus, though the visible signs that owners notice—bleeding, swelling, and behavioral changes—are most prominent in the first two weeks. Outliers exist: some females have very light or irregular bleeding, and others have longer or shorter intervals between cycles.

Inside the canine reproductive cycle — key biological facts

The canine estrous cycle has four main phases: proestrus, estrus, diestrus, and anestrus. Proestrus is when the vulva swells and bloody discharge appears; males usually show interest but females typically aren’t receptive yet. Estrus is the fertile window when females allow mating; discharge often becomes less bloody and may be pinkish or straw-colored. Diestrus follows sexual receptivity and lasts as the reproductive tract returns to baseline whether or not pregnancy occurred. Anestrus is the resting phase between cycles.

Hormones drive these changes. Rising estrogen is responsible for the external signs of proestrus and prepares the reproductive tract, while a rise in progesterone around the time of ovulation supports potential pregnancy and then dominates during diestrus. The precise timing of hormonal shifts can vary between individuals; blood testing or vaginal cytology may be used by breeders and veterinarians to estimate ovulation more accurately. Canine cycles are not like human menstruation—there isn’t a monthly shedding of the uterine lining in the same way, and the visible bleeding early in the cycle is part of proestrus rather than a menstrual period.

Behavioral cues that mating may be successful include a change in tail carriage, standing to be mounted, and reduced avoidance of males. These cues are helpful to watch, but they don’t replace veterinary guidance when planning breeding or medical care.

Seasons, size, diet and stress: factors that alter heat timing

Breed and body size are strong predictors of when puberty and the first heat happen. Smaller breeds generally mature faster, while larger breeds take longer. Nutrition and growth rate also matter; dogs that are overweight or that grow unusually quickly or slowly may show shifted timing, and undernutrition can delay onset. I often advise owners that a healthy, consistent diet supporting appropriate body condition is likely linked to more predictable maturation.

Seasonality plays a role in some breeds and populations. Wild canids and some primitive breeds may have more strictly seasonal cycles, while most domestic dogs cycle year-round. Geographic factors such as day length and climate are likely linked to subtle differences in cycle timing in some populations. Stress, illness, or medications can temporarily alter or suppress cycles; for example, severe systemic illness may delay a first heat, and certain drugs can interfere with normal hormonal patterns. If your dog’s cycle timing shifts dramatically after an illness or a change in household routine, discuss it with your vet.

Warning signs and red flags: when to seek veterinary help

Not all changes in a heat cycle are benign. Abnormal bleeding patterns—very heavy bleeding, prolonged proestrus beyond several weeks, or sudden return to bloody discharge after a quiet period—may suggest an underlying problem. Foul-smelling or pus-like vaginal discharge is not normal and may indicate infection. If you notice fever, marked lethargy, vomiting, or abdominal discomfort, these signs may point to pyometra, which is an infection of the uterus that can be life‑threatening and typically requires urgent veterinary care.

Very early puberty (cycles before six months in a medium or large dog) or a markedly delayed onset (no cycle by 18–24 months in some larger breeds) should prompt veterinary evaluation. These patterns can be linked to hormonal disorders, congenital issues, or systemic disease. If breeding is planned, recurrent irregular cycles or failed pregnancies are also reasons to consult a reproductive specialist; sometimes diagnostic testing (hormone assays, ultrasound, cultures) is recommended.

Owner checklist — practical actions to take during a heat cycle

1. Start tracking: Record the date you first notice swelling or bleeding and note behavior changes. A calendar, dedicated app, or simple notebook helps spot average intervals over time.
2. Separate during fertile periods: From proestrus onward, assume males will be attracted. Keep your female away from intact males from the first signs until after estrus ends—this may mean separate walking routes, different times of day, or temporary housing changes.
3. Contact your veterinarian: Discuss the best time to spay if that is your plan; some vets prefer waiting until the dog is out of estrus, while others advise timing for other health considerations. If you plan to breed, ask about pre-breeding health screens (brucellosis testing, genetic screening, vaccination review).
4. Hygiene and monitoring: Expect some staining and mild discharge. Keep bedding washable and check the vulva and discharge daily. Avoid douching or overcleaning the vagina—gentle external wiping with mild, pet-safe wipes is usually enough. Seek veterinary care for green, brown, or foul discharge, or significant changes in appetite and energy.
5. Prepare for behavior: Expect increased restlessness or clinginess. Provide extra exercise and mental enrichment to reduce stress, but avoid off-leash areas where intact males could reach your dog.

Keeping routine: managing the home and training while your dog is in heat

Preventing escape and accidental mating depends on consistent environmental controls. Secure fencing (check for gaps and dig-outs), supervised exits, and double-door systems for homes with busy entryways reduce risk. I often recommend a “two-person” routine for busy households—one person manages the female while the other opens doors—to avoid sudden bolting.

When walking, use a short, sturdy leash and a secure harness rather than a collar alone; a harness reduces the chance of slipping free if a male becomes aggressive or distracting. Avoid dog parks, daycares, and multi-dog playdates when your female is in heat. If you must use shared spaces, choose times when fewer dogs are present and keep your dog at a distance.

Training to reduce escape behavior pays off. Practice reliable recall and sit/stay in fenced yards, and work on impulse control around gates and doors. If your dog becomes fixated on male dogs, consult a behaviorist for desensitization and management strategies to reduce stress for both dogs and owners.

Gear guide: helpful products for managing heat cycles

• Dog diapers and washable belly bands provide reliable protection for furniture and bedding; choose breathable fabrics and check frequently to prevent skin irritation.
• Secure harnesses and reinforced leashes (no-escape or front-clip harnesses) help prevent slipping free during walks.
• Soft, pet-safe wipes and enzymatic cleaners for launderable bedding address hygiene without harsh chemicals; maintain skin folds dry to avoid secondary infections.
• Calming aids—such as veterinary-recommended pheromone diffusers or thundershirts—may help anxious dogs during the cycle, but they are adjuncts, not replacements for safe management.

If complications occur: common problems and recommended next steps

If you suspect pyometra—fever, vomiting, drinking more or less than usual, abdominal swelling—treat it as urgent. Many cases require surgery and antibiotics. If bleeding is excessive or continuous beyond what you’ve observed in past cycles, bring your dog in for an exam. For early or delayed puberty, a vet will likely review growth, nutrition, and run basic bloodwork or imaging. If you plan to breed and encounter infertility or repeat pregnancy loss, your veterinarian may refer you to a reproductive specialist for targeted testing.

After a heat cycle, consider whether spaying is appropriate for your dog’s health and lifestyle; I often discuss the benefits and timing in the context of the dog’s age, breed-specific risks (like certain cancers), and behavior. If you’re uncertain, schedule a pre-spay consultation to discuss individualized risks and benefits.

References and expert sources

• Merck Veterinary Manual. “Estrus (Heat) in the Dog.” Merck Veterinary Manual, latest edition. https://www.merckvetmanual.com
• American Veterinary Medical Association (AVMA). “Spaying and Neutering.” AVMA Guidelines and Client Resources. https://www.avma.org/resources-tools/pet-owners
• American Animal Hospital Association (AAHA). “AAHA Canine Reproduction Guidelines.” AAHA Guidelines for veterinary practice teams, 2017–2019.
• Johnston SD, Kustritz MV, Olson P. Canine and Feline Theriogenology. 2nd ed., Saunders, 2001. (Textbook on small animal reproduction)
• Concannon PW. “Clinical aspects of the canine estrous cycle.” Veterinary Clinics of North America: Small Animal Practice, Reproduction and Theriogenology sections.