How do you know when a dog is in heat?

When a female dog goes into heat it changes daily routines, affects the safety of walks, and can create health risks if you aren’t prepared. For owners of intact females, new adopters who don’t know the dog’s history, and households near roaming males, recognizing heat signs helps prevent unwanted pregnancies and reduces stress for everyone involved. I see this most often when owners are surprised by male dogs showing up at the fence or when a young dog behaves differently during a family outing.

Understanding heat is also practical: it influences decisions about when to spay, whether to try breeding, and how to arrange boarding or daycare. Simple awareness can avoid an accidental litter, reduce vet bills from infections that sometimes follow cycles, and keep a dog’s emotional well‑being steadier through predictable changes.

How to tell — fast: clear signs your dog is in heat

Here’s a short checklist you can use immediately if you suspect your female dog is in heat. If several of these appear together, she is likely cycling and you should act accordingly:

• Visible signs: a swollen vulva with blood‑tinged or rusty/discolored discharge that may start heavier then thin out.
• Behavioral signs: increased attention from male dogs, more frequent urination to mark territory, restlessness, unusually clingy or, conversely, more independent behavior; mounting or “flagging” where she lifts her tail to one side when receptive.
• Timing overview: heat usually begins with a proestrus phase (swelling, bleeding) and moves into estrus when she is most likely to conceive; the fertile window is often a few days to about a week within estrus but can vary by individual.

Inside the cycle: the hormones and biology behind heat

Dogs go into heat as part of the estrous cycle, a repeating hormonal process whose goal is reproduction. The cycle commonly divides into stages that usually include proestrus, estrus, diestrus, and anestrus, each with different physical and behavioral signs. Proestrus is when estrogen levels rise and you are likely to see vulvar swelling and bloody discharge; estrus is when progesterone rises and the female becomes receptive to males.

These hormonal shifts are likely linked to changes in the uterus and ovaries that make pregnancy possible. Estrogen may be driving the outward signs that attract attention, while progesterone supports ovulation and prepares the body for a potential pregnancy. Although the timing follows a general pattern, the exact length and intensity of each stage often varies by dog.

When it happens: typical timing, age and frequency of heat cycles

First heats most often appear between about six and twenty‑four months of age, but smaller breeds commonly cycle earlier (often around six to nine months) while large breeds sometimes don’t have their first heat until a year and a half or later. I typically tell owners to expect variability: some dogs follow a roughly six‑month rhythm, but intervals can range from four to twelve months or longer in particular breeds.

Unlike many wild canids that are seasonal, domestic dogs may come into heat at any time of year. Some owners notice seasonal clustering, but many do not; household life, nutrition, and individual biology all appear to influence timing. Breed differences are clear: toy and small breeds often cycle more frequently than giant breeds, and individual dogs within a breed can still differ markedly in cycle length and signs.

Medical warning signs: symptoms that require immediate veterinary attention

Most heats resolve without medical treatment, but certain signs suggest complications and warrant prompt veterinary attention. Abnormally heavy or prolonged bleeding that soaks bedding or diapers for more than a few days may indicate a problem, as unusually heavy blood loss is not typical for the usual proestrus pattern.

Discharge that smells foul, becomes pus‑like, or turns greenish or yellowish may suggest infection, including a serious uterine infection called pyometra that is often linked to hormonal changes after heat. Systemic signs such as fever, vomiting, loss of appetite, marked lethargy, or abdominal pain are red flags and should prompt immediate evaluation.

Sudden extreme behavioral changes—aggression, apparent severe pain when touched, or collapse—also require urgent assessment. If you’re uncertain whether a sign is normal for this dog, keeping notes on timing, appearance, and behavior can help your veterinarian decide how quickly to act.

Owner action plan: practical steps to take during a heat cycle

1. Record and track dates: note the first day you see bleeding, changes in behavior, and when the discharge changes color or stops. This creates a timeline you can share with your vet and helps predict future cycles.
2. Prevent contact with intact males: keep the dog on leash during walks, avoid off‑leash areas and dog parks, and supervise outdoor time in a securely fenced yard; males can be very persistent and may try to jump fences or dig under them.
3. Maintain hygiene: use washable bedding and clean any soiled areas promptly with mild, pet‑safe cleaners; for the dog, gentle wiping with a damp cloth is usually enough—avoid douches or human products unless your vet recommends them.
4. Know when to call the vet: contact your clinic if bleeding is heavy or prolonged, the discharge has a bad odor or pus, if the dog shows fever, vomiting, refuses food, or appears to be in pain, or if you’re considering breeding and want timing advice or health testing.

At-home care and training: keeping your dog comfortable and safe

Practical adjustments make heat cycles easier for both dog and owner. I advise short, predictable walks on a secure leash at quieter times of day to reduce encounters with male dogs. Change socialization plans: postpone dog parks and group playdates until the cycle is over, as the risk of unwanted mating is high and other dogs may react unpredictably.

Keeping training sessions short and reward‑based can reduce anxiety. Mental enrichment—puzzle feeders, scent games, short obedience drills—helps focus her attention and burns energy without long outings. If she seems restless at home, scheduled play or grooming can be calming.

When other pets are in the house, manage household dynamics by separating sleeping areas when needed and supervising interactions with male dogs. Intact male household mates may become more interested and agitated; consider temporary separation or extra exercise to lower tension.

Must-have supplies: practical gear for managing heat

There are a few simple items that make a cycle easier to manage. Dog diapers or period pants with absorbent pads help keep the house clean and reduce stress when bedding or furniture gets soiled. Waterproof bedding covers and easily washable blankets are worth having on hand.

For outdoor safety, secure harnesses and sturdy leashes are important, as are tall, solid fencing and gates to prevent determined males from getting in. Calming aids like pheromone diffusers, snuffle mats, and puzzle toys can ease anxiety; if severe stress persists, consult your vet before using supplements or medications.

Breeding or spaying? Options, timing and what to discuss with your vet

If you’re considering breeding, talk with a veterinarian experienced in reproduction early. Accurate timing for mating is often based on observing behavior and discharge, and some breeders use progesterone testing or vaginal cytology to pinpoint ovulation. Health screening before breeding—hip/elbow evaluations, genetic testing, and general wellness checks—is important for the welfare of the bitch and future puppies.

If you plan to spay, timing matters. Many vets recommend waiting until after the first heat for certain breeds, while others advise spaying earlier or later based on health, behavior, and cancer risk factors. I typically discuss the individual dog’s breed, age, and lifestyle to decide the best timing; talk to your vet about pros and cons for your situation.

Troubleshooting complications: what to do if something goes wrong

If you notice signs of infection, systemic illness, or severe behavioral change, seek veterinary care promptly. Pyometra can progress quickly and is potentially life‑threatening; early treatment—often surgery—is likely to be lifesaving. A vet will evaluate your dog with an exam, bloodwork, and possibly imaging, and will advise next steps based on findings.

For behavioral issues that emerge or worsen during heat, a behavior consultation can help. Temporary management strategies, such as stricter confinement, training adjustments, or short‑term medication, may make a cycle tolerable while you work toward a longer‑term plan like spaying if appropriate.

Sources and further reading

• Merck Veterinary Manual: Canine Estrous Cycle and Breeding Management
• Merck Veterinary Manual: Pyometra in Dogs
• American Veterinary Medical Association (AVMA): Spaying and Neutering — Benefits and Considerations
• VCA Animal Hospitals: Heat in Dogs (Estrus) — Signs and Care
• American Kennel Club (AKC): When Do Dogs Go Into Heat? Timing, Stages, and What to Expect