When do female dogs start their period?

Many dog owners notice their female pet’s first “period” with surprise or concern. Knowing when heat typically begins, what it looks like, and how to manage it can keep your home calm, protect your dog’s health, and help you make informed choices about breeding or spaying. Below I lay out what I see in practice, why it happens, and exactly what to do from first spotting blood to planning veterinary care.

How her first cycle affects care, behavior and training

When a female dog comes into heat it affects more than her body: it changes daily routines, interactions with other dogs, and sometimes family dynamics. Household planning is practical—young dogs in heat may need temporary separation from intact males, extra cleaning, and more supervision outdoors. For owners thinking about breeding, timing matters: missing the right window can mean a lost opportunity or unintended pregnancy. Children and other family members often need a simple explanation and clear rules so they don’t unintentionally allow contact with neighborhood dogs. And even if you don’t plan to breed, deciding when to spay is a family decision that benefits from knowing the first heat timing and typical cycle pattern.

At a glance: when most female dogs have their first period

Most female dogs have their first heat somewhere between about 6 and 24 months of age. Smaller breeds often enter heat toward the earlier end of that range; some toy breeds may show signs at 4–6 months, while many large or giant breeds commonly start closer to 18–24 months. The bleeding you see is part of the estrous cycle and is not the same as human menstruation—dogs shed a small amount of blood during the proestrus phase, which may last roughly 7–10 days. After that, there is typically a fertile window when mating can result in pregnancy. Most intact females come into heat about twice a year, but the exact frequency can vary from three times a year to once every 12 months depending on the individual.

What’s going on biologically during heat

A dog’s heat cycle exists to create a fertile window for reproduction. The cycle moves through recognizable stages: proestrus (when bleeding and behavioral changes start), estrus (the period of sexual receptivity and ovulation), diestrus (when progesterone rises and the body behaves as if pregnant whether mating occurred or not), and anestrus (a resting phase). Hormones drive these shifts—estrogen rises during proestrus and is likely linked to the swelling of the vulva and bloody discharge, while progesterone becomes dominant after ovulation and is associated with the body preparing to support pregnancy. You’ll see behavioral signs that match those hormonal changes: more attention-seeking or restlessness during proestrus, tolerance or active receptivity to males during estrus, and then a return to baseline in anestrus. I typically see owners focus on the visible bleeding, but that bleeding is only one outward clue to a complex hormonal sequence beneath the surface.

Factors that influence the age of a dog’s first estrus

Breed and genetics are strong influences: small-breed dogs tend to mature faster and enter puberty earlier, while large-breed dogs take longer to reach sexual maturity. A dog’s overall growth rate and body condition are also important—underweight or malnourished animals may show delayed first heats, whereas well-fed, rapidly maturing dogs may come into heat earlier. Household factors can play a role too; the presence of intact males nearby may alter behavior and stress levels, and living in a multi-dog household can shift the timing of observed signs. Seasonal and environmental cues are less rigid in domestic dogs than older domestic species, but some owners notice patterns tied to light cycles or changes in routine; such observations may suggest an influence rather than prove one. Because of this variation, comparing your dog to breed averages is helpful but not definitive.

When to contact your vet: warning signs and health risks

Not every change during heat is normal. If bleeding is heavy, lasts much longer than two to three weeks, or is accompanied by an unpleasant smell, that may suggest infection or another problem and deserves veterinary attention. Fever, ongoing vomiting, refusal to eat, or severe lethargy are clear warning signs. One serious risk after a heat cycle—especially in older, unspayed females—is pyometra, an infection of the uterus that can be life-threatening; symptoms can include a swollen abdomen, foul discharge, fever, and sudden changes in behavior. Other red flags include very painful or hot swelling of the vulva, dark or pus-like discharge, or any signs that the dog seems in pain when moving. If you see those signs, seek veterinary care promptly rather than waiting to see if they resolve.

Owner checklist: preparing for and responding to her first heat

1. Start by tracking cycle dates and symptoms in a notebook or an app. Note the day you first see blood, changes in appetite or behavior, and when receptivity to males begins. This record makes future timing and veterinary conversations more precise.
2. Limit exposure to intact males. Keep your dog supervised and leashed during walks, avoid dog parks, and consider temporary separation within the household if you have an intact male. Even distant males can be persistent when scent is present.
3. Monitor the amount and character of the discharge. Small, serosanguineous (pinkish) discharge for about a week can be normal, but prolonged or foul-smelling discharge needs a vet check. Call your veterinarian if bleeding is unusually heavy or lasts beyond three weeks.
4. Decide on spaying versus planned breeding before the heat arrives if possible. If you plan to spay, many veterinarians recommend waiting until after the first heat, while others favor spaying before heat to reduce certain health risks—discuss timing with your vet based on breed and health. If you plan to breed, work with a breeder or reproductive specialist to time mating and screen both dogs for health issues.
5. Schedule veterinary care for any abnormalities or to plan elective spay. Bring your cycle notes to the visit to help the vet advise you about timing, risks, and any diagnostics that may be needed.

Keeping your home, routines and training on track during season

Practical home management reduces stress for everyone. Set up a confined, comfortable area with washable flooring or covers that can be changed daily; many owners use a gated bedroom or a wire crate with secure bedding that’s easy to launder. Modify walks: keep your dog on a short, secure leash, choose quiet routes, and avoid off-leash areas until bleeding and receptivity have ended. Training can help reduce marking and roaming impulses—rehearse recall at short distances, reward calm behavior, and avoid situations where your dog has the chance to sprint toward an intact male. In multi-pet homes, introduce or separate animals slowly; some intact males may become obsessed by scent and repeatedly attempt to access the female, so physical barriers and supervision are necessary. If you have children, give them a straightforward rule: no unsupervised handling and explain how to stay safe and calm around the dog while she’s in heat.

Recommended supplies to manage heat safely and comfortably

There are several practical items that make the heat cycle easier to manage. Reusable or disposable dog diapers and sanitary wraps help contain discharge and protect furniture; choose breathable designs sized to your dog and change them frequently to reduce the risk of skin irritation. Washable bedding and protective couch covers allow you to isolate areas for sleep and lounge time without constant carpet cleaning. For walks, use secure harnesses and short, escape-proof leashes so the dog can’t slip free if startled by another dog. For comfort, have calming aids and distraction toys on hand—long-lasting chew toys, food puzzles, or penicillin-free (i.e., not medicinal) calming wraps can reduce anxiety during hormonal changes. Keep grooming supplies nearby: gentle pet-safe wipes and a soft towel make quick cleanups easier and more comfortable for your dog.

References and further reading

• Merck Veterinary Manual. “Estrous Cycle (Heat) in Dogs.” Merck Veterinary Manual: Canine Reproduction section.
• Johnston SD, Root Kustritz MV, Olson PNS. Canine and Feline Theriogenology, 2nd edition. (Textbook chapter on the canine estrous cycle).
• Feldman EC, Nelson RW. Canine and Feline Endocrinology and Reproduction, 4th edition. (Chapters on ovarian function and reproductive management).
• American Veterinary Medical Association. “Spaying and Neutering: Information for Pet Owners” and related guidance on timing and health implications.
• Concannon PW. “Reproductive cycles of the domestic dog.” Journal of Reproduction and Fertility. (Review of hormones and timing in the canine cycle).