What are the 4 stages of a dog in heat?

If you care for a female dog—or share neighborhoods and dog parks with intact animals—understanding the four stages of a dog in heat can make the difference between a smooth, safe cycle and an emergency trip to the clinic. This guide explains what each stage looks like, why it happens, what you should do in each phase, and when to call your veterinarian.

How the heat cycle affects your dog — and your home

Knowing the stages of heat helps you protect your dog’s health and reputation, plan breeding or spay timing, and reduce risky interactions with other dogs. For first-time owners, anticipating the first heat avoids surprise messes and unsafe escapes. For breeders, accurate timing is central to deciding whether to breed or avoid mating. If you live in a busy neighborhood or walk an intact male, the scent and behaviors of a female in heat can trigger strong, sometimes dangerous reactions—understanding the cycle lets you prevent those encounters. Pet sitters, walkers, and daycare operators also benefit: a clear plan reduces liability and keeps group settings calm and sanitary.

I typically advise owners to treat the heat cycle as both a biological process and a predictable event you can plan for. Mark start and end dates, tighten supervision, and discuss breeding intentions or spay timing with your vet well before the expected window.

At a glance: the four stages of a dog in heat

• Proestrus — the bleeding and attraction phase, usually about 3–17 days. The vulva swells and there’s often a bloody discharge; males notice her but she may not yet accept them.
• Estrus — the fertile period when ovulation occurs, generally about 3–14 days. Discharge often lightens; the female may allow mating and shows receptive behaviors.
• Diestrus (metestrus) — the luteal or post-ovulatory phase lasting roughly 60–90 days. Progesterone is dominant whether or not pregnancy occurred; behavior and tissue changes follow ovulation.
• Anestrus — the reproductive rest between cycles, variable in length (often several months) before the next proestrus begins.

What triggers a dog’s heat cycle: hormones, age and behavior

The heat cycle exists to enable reproduction, driven by coordinated rises and falls in reproductive hormones. A rise in estrogen usually initiates proestrus and produces the swelling and behavioral signs you notice. Near the end of estrus, a surge in luteinizing hormone (LH) is likely linked to ovulation; this is the narrow window when eggs are released and mating can result in pregnancy. After ovulation, progesterone becomes the dominant hormone during diestrus and supports a potential pregnancy or maintains a non-pregnant luteal phase that still affects behavior and body systems.

Along with hormonal changes, pheromones and behavioral signals are released that attract males and signal fertility. The strength and timing of these signals may vary between dogs; some show obvious changes, others subtler ones. Environmental cues such as light and season may also nudge timing, especially in breeds with seasonal patterns.

Timing and triggers: factors that influence when heat begins

Many dogs first cycle around six months of age, but “first heat” commonly ranges from about four months to over 18 months depending on size and breed. Small-breed dogs frequently come into heat earlier than large-breed dogs; for example, a small terrier may cycle near five to seven months, while a giant-breed dog might not have its first heat until 12–18 months.

After the first heat, average inter-estrus intervals across most pet dogs are roughly six months, but that varies. Some breeds—Basenjis, for instance—tend to cycle once a year, while toy breeds might cycle more frequently. Nutrition, body condition, health issues, and endocrine disorders such as hypothyroidism or ovarian cysts may alter timing or cause irregular cycles.

Season and daylight length can influence reproductive hormones in some animals; while the domestic dog is less strictly seasonal than some species, you may still see subtle seasonal patterns in certain breeds or in dogs living at higher latitudes. Stressful household changes, sudden weight loss, or poor nutrition may delay or suppress cycles; conversely, improved condition can restore regular cycling.

Risks and red flags: health issues to watch for

Most heats are uncomplicated, but several signs should prompt immediate veterinary attention. Pyometra—an infection of the uterus—is a serious, potentially life-threatening condition that often appears after heat. Warning signs may include a sudden, foul or brown vaginal discharge, fever, lethargy, increased drinking and urination, abdominal swelling, or loss of appetite. If a dog develops any of these signs after a heat, seek urgent veterinary care.

Excessive or prolonged bleeding beyond the usual proestrus window, very heavy clots, or bloody discharge that changes in smell or color may suggest a problem. Severe behavioral changes—intense aggression, persistent signs of pain, or marked lethargy—are also reasons to get a veterinary assessment. Repeated irregular cycles, failure to come into heat when expected, or sudden infertility despite normal mating history may indicate endocrine or reproductive tract issues that deserve diagnostic testing.

Owner’s checklist: what to do during each stage

• Track dates and signs: write down the day you first see bleeding or swelling, note changes in discharge and behavior, and record when receptive or unreceptive behaviors begin and end. These notes are invaluable for breeding decisions and veterinary evaluations.
• Prevent unwanted mating: plan supervised confinement during yard time, use double-locked gates and secure crates for transport, and never leave an intact female unsupervised with intact males—males can be extremely persistent and may chew barriers or dig to reach her.
• Maintain hygiene: use well-fitted dog diapers or reusable pads to protect furniture and bedding; change pads frequently, wash bedding with enzymatic cleaners to remove scent, and keep the vulvar area clean and dry to reduce irritation or secondary skin issues.
• Consult your veterinarian: if you’re considering breeding, schedule pre-breeding exams and discuss timing and genetic screening. If you’re not breeding, discuss the best time for spay surgery and any health monitoring your dog might need during and after heat.

Managing walks, training and household routines while she’s in heat

Practical adjustments at home reduce escape and unwanted encounters. Inspect fencing for gaps, add latches to gates, and consider temporary barriers if you live in an area with many intact males. When walking, prefer quiet routes and off-peak times, keep your dog on a short leash, and use a secure harness instead of a collar to reduce the risk of slipping loose. Double-leash routines—one person holding two points of connection—add security during higher-risk moments.

Pause group activities: suspend dog park visits, group playdates, and doggy daycare unless the facility is prepared to manage intact females. Reinforce recall and Calmness training before walks; I usually recommend short, reward-based sessions focused on leash manners and settling. Let neighbors, dog walkers, and household members know the dates so everyone can take consistent precautions. If your dog is anxious or restless, increase mental enrichment—food puzzles, scent games, and controlled training sessions can help redirect energy without encouraging roaming.

Essential gear and supplies to make heat easier to handle

Simple, reliable gear makes the cycle easier for you and your dog. Well-fitting dog diapers and washable pads reduce mess and protect furniture; choose breathable materials and check frequently for skin irritation. Protective garments that cover the vulva help keep diapers in place for active dogs. For outings, use short, sturdy leashes and harnesses that minimize the chance of escape; travel crates or secure carriers are the safest way to transport a dog in heat around unfamiliar dogs.

Keep enzymatic cleaners and odor-neutralizing products on hand for bedding, carpets, and car upholstery—enzymatic formulas help break down scent molecules that attract male dogs. If your dog becomes anxious, pheromone diffusers or sprays designed to promote calm may help some individuals; speak with your vet before trying supplements or medications so you choose safe, appropriate options for your dog’s health and any existing conditions.

Breeding or spaying? How to decide and the next steps

If you intend to breed, plan well ahead: pre-breeding exams, infectious disease screening, and timing based on vaginal cytology or progesterone testing improve the chance of successful mating and healthy puppies. I typically recommend working with an experienced breeder or a reproductive specialist who can advise on optimal breeding windows and health checks.

If you do not want puppies, spaying in a non-pregnant period is the most definitive option and often recommended between heats once you’ve discussed timing with your veterinarian. Spaying reduces the risk of pyometra and eliminates future heats; timing can vary by age, breed, and health status, so plan the procedure when your dog is healthy and not actively bleeding unless advised otherwise by your vet.

References and further reading

• Merck Veterinary Manual: “Canine Estrous Cycle” — https://www.merckvetmanual.com/management-and-nutrition/physiology-of-reproduction/canine-estrous-cycle
• American Veterinary Medical Association (AVMA): “Spaying and Neutering” and owner guidance on reproductive health — https://www.avma.org/resources-tools/pet-owners/petcare/spaying-and-neutering
• American Kennel Club: “When Do Dogs Go Into Heat?” and breed-specific notes on cycle timing — https://www.akc.org/expert-advice/health/when-do-dogs-go-into-heat/
• Johnston SD, Root Kustritz MV, Olson P: Canine and Feline Theriogenology (textbook) — standard veterinary reproduction reference
• Concannon PW, England GCW, Verstegen JP: “Recent advances in small animal reproduction,” Theriogenology and journal articles on canine ovarian and hormonal physiology