Dog Heat Cycle: Signs, Duration & What to Expect

By Sarah Bennett, Certified Animal Nutritionist

Every intact female dog goes through a reproductive cycle called the estrous cycle — commonly referred to simply as "heat" or "season." Understanding this cycle is essential whether you're a responsible pet owner managing an intact dog, a breeder timing a planned mating, or someone considering spaying. The canine heat cycle is more complex than many people realize, consisting of four distinct phases with very different hormonal profiles and behavioral implications.

The Four Stages of the Dog Heat Cycle

Stage 1: Proestrus (7–10 Days)

Proestrus is the beginning of the visible heat cycle. Estrogen levels rise rapidly as follicles develop on the ovaries. The most obvious external sign is vulval swelling and a bloody vaginal discharge. The blood-tinged discharge results from increased blood vessel permeability in the uterine lining — it is normal and not a sign of injury or illness.

Despite being clearly attractive to male dogs (who will follow her, attempt to mount, and be intensely interested), the female in proestrus is not yet receptive to mating. She will typically sit down, move away, or snap at males who attempt to mount. This distinction is important for breeders — a female in proestrus will not stand for mating and the timing would generally be too early for conception.

Proestrus lasts an average of 7–10 days, though the range extends from 3 days to 3 weeks in individual dogs.

Stage 2: Estrus (5–9 Days)

Estrus is the period of true fertility. As estrogen reaches its peak and then triggers an LH (luteinizing hormone) surge, ovulation occurs — typically around day 9–14 of the cycle from the start of proestrus. The vaginal discharge lightens in color, shifting from red to a pale straw or pinkish hue. The vulva may become slightly softer and less swollen.

The defining behavioral change of estrus is flagging: the female deflects her tail to one side when approached by a male, signaling receptivity and willingness to mate. She will actively approach and stand for mounting. This behavioral shift is the clearest practical signal that she has entered her fertile window.

Estrus lasts an average of 5–9 days. Since canine sperm can survive in the reproductive tract for up to 7 days, and eggs are viable for 48–72 hours after ovulation, the fertile window is relatively broad — though optimal conception rates are achieved when mating coincides with ovulation. Progesterone testing by a vet can pinpoint this precisely.

Stage 3: Diestrus (60–90 Days)

Diestrus begins when the female is no longer receptive to mating, regardless of whether conception has occurred. This phase is hormonally identical in pregnant and non-pregnant dogs — progesterone remains elevated for 60–90 days in both cases. This is why phantom pregnancy is so common: the body cannot distinguish a real pregnancy from diestrus following an unmated heat cycle.

Externally, the vulva returns to its normal size and discharge ceases. The dog behaves normally. If she is pregnant, the embryos are developing during this period. If she is not, the uterus undergoes normal post-heat changes that may, in some older intact bitches, lead to cystic endometrial hyperplasia (CEH) — a precursor to the life-threatening infection pyometra.

Stage 4: Anestrus (The Rest Phase)

Anestrus is the period of reproductive quiescence between cycles. Hormones are at their lowest levels, the reproductive tract is resting and recovering, and the dog shows no signs of heat and is not attractive to males. Anestrus lasts an average of 4–5 months in most dogs, giving a total cycle (proestrus through anestrus) of approximately 6–8 months.

Giant breeds such as Great Danes, Saint Bernards, and Irish Wolfhounds may cycle only once per year, with anestrus lasting 8–10 months. This is normal for these breeds and not a sign of reproductive dysfunction.

Signs Your Dog Is in Heat

The recognizable signs of an active heat cycle include:

• Vulval swelling: Often the first sign owners notice, beginning in proestrus.
• Vaginal discharge: Bloody in proestrus, lightening to straw-colored in estrus.
• Increased urination: The dog marks territory more frequently; urine contains pheromones that attract males from a significant distance.
• Behavioral changes: Increased restlessness, clinginess, or conversely unusual independence. Some dogs become more vocal.
• Attracting male dogs: Males may gather at your gate or follow on walks — sometimes aggressively.
• Flagging: Tail deflection when touched on the rump — the clearest sign of full estrus and receptivity.
• Excessive grooming: Self-licking of the vulval area.

Duration and Frequency of the Heat Cycle

The active phase (proestrus plus estrus) lasts an average of 2 to 4 weeks total. Most dogs cycle approximately every 6 months, meaning two heat cycles per year. However, individual variation is wide. The first heat cycle in a young dog typically occurs between 6 and 12 months of age in small breeds, and between 12 and 24 months in giant breeds. It is advisable not to breed during the first heat — the dog is not yet physically or behaviorally mature.

Managing a Dog in Heat

If you have an intact female who is not intended for breeding, managing her heat cycle requires vigilance. Even brief unsupervised encounters with intact male dogs can result in mating — and dogs are persistent and creative in their pursuit. Keep her on-lead at all times outdoors, avoid dog parks during estrus, and be aware that male dogs may attempt to enter your garden or home. Intact males can detect a female in heat from over a mile away.

Protective garments (dog heat pants or nappies) can help contain discharge indoors, though they don't prevent mating if a dog slips through a door. Some owners find that certain calming supplements help reduce restlessness and anxiety during the hormonal peak, though these do not affect fertility or the cycle itself.

Health Considerations and When to Spay

Every heat cycle in an intact female carries a small additional risk of pyometra — a severe uterine infection that is life-threatening without surgery. The risk increases with each cycle and accelerates in middle-aged and older intact bitches. Pyometra has a cumulative lifetime incidence of approximately 25% by age 10 in intact bitches. Spaying eliminates this risk entirely.

The optimal timing for spaying depends on breed, size, and individual health factors — consult your vet for personalized guidance. For most small-to-medium breeds, spaying before the first or second heat is commonly recommended. For large and giant breeds, later neutering (18–24 months) may be advisable to allow musculoskeletal development to complete.