What age do dogs go into heat?

Most dog lovers eventually want a clear answer to the simple question, “When will my dog go into heat?” That timing affects daily care, breeding choices, and decisions about spaying. The answer isn’t one-size-fits-all, but you can plan effectively if you understand the typical ages, what drives the cycle, and practical steps to manage it safely.

Why Knowing When Your Dog Will Go Into Heat Matters

Seeing staining or blood on a first-time female can be alarming for a new owner; many people interpret it as injury or illness. For others, knowing the timing is essential for responsible breeding decisions—both to avoid accidental litters and to plan intentional matings with appropriate health screening. Timing also matters if you’re weighing when to spay: veterinarians often recommend different ages depending on breed and health concerns. In multi-dog households or neighborhoods with intact males, predictability helps prevent stress, fights, and unwanted pregnancies by letting you make practical arrangements in advance.

At What Age Do Most Dogs Have Their First Heat?

There are broad age ranges that most owners can expect, though individual dogs often fall outside them. Small breeds commonly have their first heat at around 6–12 months. Medium breeds are often ready at roughly 7–10 months. Large and giant breeds frequently mature later, with first heats commonly between 9 and 18 months. After the first cycle, many bitches cycle about every 6–9 months, but the interval can be shorter or longer depending on the dog.

• Small breeds: often 6–12 months
• Medium breeds: often 7–10 months
• Large/giant breeds: often 9–18 months

The Biology of a Canine Heat Cycle: Hormones and Stages

Heat, or the estrous cycle, is driven by a chain of hormones that starts in the brain and ends in the reproductive tract. A gland in the brain releases signals that stimulate the pituitary to release luteinizing hormone (LH) and follicle-stimulating hormone (FSH). Those hormones prompt the ovaries to produce estrogen and later progesterone, which in turn change the uterus and behavior in ways that enable ovulation and possible pregnancy. The cycle has stages usually called proestrus (when bleeding and swelling begin), estrus (when the female is receptive to males), diestrus (a luteal phase after ovulation), and anestrus (a quieter period). Behavioral changes—such as more rubbing, flagging the tail, or allowing male attention—are normal signals that a female is moving through these stages and is likely to attract male dogs.

What Determines Timing — Breed, Size, Health and Environment

Breed and genetics are the largest predictors of when the first heat arrives; small breeds have a faster growth tempo and more frequently come into heat earlier than giant breeds, which mature more slowly. Body condition and nutrition can shift timing: underweight or malnourished dogs may mature later, while very overweight dogs may show irregular cycles. Certain health problems, like hypothyroidism or other endocrine disorders, may change cycle timing or make signs less obvious. While many dogs don’t show strict seasonality, environmental factors—such as living in a household with intact males or exposure to artificial lighting—may influence cycle regularity. Finally, early spay/neuter, prior hormonal treatments, or trauma to the reproductive tract can alter or prevent normal cycles.

Warning Signs: When a Heat Cycle Needs Veterinary Attention

Most heats are inconvenient but not emergencies. However, some signs suggest a problem that needs prompt veterinary attention. Heavy or prolonged bleeding beyond the usual 2–3 weeks, or bleeding that becomes heavier rather than tapering, may indicate an infection or other issue. A foul odor or pus-like vaginal discharge suggests infection and requires evaluation. Systemic signs—fever, severe lethargy, not eating, vomiting, or abdominal pain—are worrying because they may point toward pyometra, a life-threatening uterine infection that can occur in the weeks following estrus. Sudden and persistent swelling of the vulva, or return to heat very shortly after a cycle, may suggest ovarian cysts or hormonal imbalances. If you notice any of these, contact your veterinarian without delay.

What to Do First When Your Dog Starts Her Heat

1. Start tracking. From the first sign of bleeding or vulvar swelling, note the date, duration, amount of discharge, and behavioral changes; this record will help you predict the next cycle and communicate clearly with your veterinarian.
2. Discuss spay timing. Talk with your veterinarian about the best age to spay based on breed, size, and health. I typically recommend individualized discussion rather than a single blanket age because risks and benefits shift with breed and lifestyle.
3. Prevent mating. Assume a female in heat can attract males from a distance; supervise all outdoor time, use secure fencing, and keep her on a leash and harness on walks.
4. Watch for red flags. Seek veterinary care for heavy or foul-smelling discharge, fever, or marked behavioral change—any signs that suggest infection or systemic illness.
5. Plan logistics. Arrange separate spaces for intact males and females, and let family members, sitters, or boarders know she’s in heat so they can take appropriate precautions.

Keeping the Household Calm: Managing Family, Other Pets and Hygiene During Heat

Practical precautions reduce stress and risk. Make sure yard gates and fencing are secure—males can be unusually persistent and may try to dig, jump, or squeeze through small openings. Keep walks short and always on a non-retractable leash with a well-fitted harness; I find that harnesses give better control when a curious male approaches. Avoid dog parks and daycare where unknown intact males could be present. Provide a quiet, comfortable space indoors where she can rest away from male dogs and loud activity. Expect temporarily increased attention-seeking or irritability; treat her calmly and avoid scolding, since stress can make the cycle feel more difficult for both of you.

Essential Supplies and Helpful Gear for a Dog in Heat

Simple items can cut down on mess and make life easier. Reusable dog diapers or sanitary pants made to fit the breed are helpful for protecting furniture during proestrus when bleeding is present; use breathable, washable types and change them frequently to avoid skin irritation. Keep absorbent, washable bedding and a waterproof pad handy for crate or bed surfaces. Bring a secure leash, a well-fitting harness, and a sturdy transport crate for vet trips or controlled outings. If you need to separate dogs indoors, lightweight playpens or pet gates can create a secure barrier without isolating her completely; ensure she still gets supervised exercise and interaction so she does not become anxious.

Guidance from Veterinarians and Professional Organizations

For authoritative guidance, consult your primary-care veterinarian first—she or he knows your dog’s history and can tailor advice. For complex reproductive questions, veterinary reproductive specialists (theriogenologists) have training in breeding management, fertility testing, and surgical timing. Breed clubs and experienced breeders can offer breed-specific observations about typical maturation and temperament during heat, which may help with planning. For balanced, evidence-informed policies on spay and neuter timing, organizations such as the American Veterinary Medical Association often provide position statements and client resources that can help you weigh options.

Thinking of Breeding? Timing, Testing and Responsible Practices

If you’re considering breeding, start early with pre-breeding health screening. I recommend health clearances for inherited conditions common in the breed and a reproductive exam before the first planned mating. Timing for mating often focuses on detecting the optimal day or two around ovulation—vaginal cytology, progesterone testing, and experienced clinical judgment are commonly used to pinpoint that window. Working with an experienced reproductive veterinarian or certified breeder can reduce wasted breedings and protect the female’s health. Remember that breeding carries responsibilities for the dam’s welfare, the puppies’ health, and placement of offspring; breed-specific clubs often have resources and recommended practices.

Considering Spaying: When to Do It and What to Expect

Choosing when to spay depends on breed, size, and health goals. Early spay reduces the risk of unwanted pregnancy and virtually eliminates the risk of pyometra later in life, but some large-breed dogs may have a slightly higher risk of certain orthopedic issues if spayed very early; the balance varies by breed. I typically encourage owners to discuss breed-specific data with their veterinarian. If you choose to wait until after one heat, be aware that behavior and mammary tissue change slightly after estrus. Your vet can outline the risks and benefits in plain terms so you can make a practical decision for your dog’s long-term health and your household situation.

References and Trusted Sources

• Merck Veterinary Manual: “Canine Estrous Cycle and Breeding Management” — Merck & Co., Inc.
• American Veterinary Medical Association (AVMA): “Spaying and Neutering of Dogs” — client information and position statements
• American College of Theriogenologists (ACT): “Canine Reproduction Guidelines and Resources” — clinical resources for breeders and veterinarians
• Johnston SD, Kustritz MV, Olson PNS. Canine and Feline Theriogenology — a clinical textbook covering reproductive physiology and management
• Cornell University College of Veterinary Medicine: “Canine Reproduction & Breeding Management” — client and clinician resources