What is Dr. Zubair Khalid's research focus?

Dr. Zubair Khalid specializes in molecular virology, mRNA vaccine development, and computational biology, with a focus on avian pathogens like IBDV and Avian Reovirus.

Where is Dr. Zubair Khalid currently working?

Dr. Zubair Khalid is a Postdoctoral Research Associate at the University of Maryland (UMD), specifically within the Department of Animal and Avian Sciences.

Estrus in Dogs: Heat Cycle Stages Explained

Name: Estrus in Dogs: Heat Cycle Stages Explained
Creator: Dr. Zubair Khalid

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-09-20

The canine heat cycle has four stages: proestrus, estrus, diestrus and anestrus. Proestrus lasts about 7 to 10 days and brings vulval swelling and a bloody discharge, estrus lasts about 5 to 9 days and is the only stage when a dog will stand to be bred, diestrus lasts roughly 60 to 90 days whether or not the dog is pregnant, and anestrus is the quiet 4 to 5 month interval before the next cycle begins [1][2].

A complete dog heat cycle therefore runs about 5 to 8 months in most bitches, and the average interval between one heat and the next is roughly 6 months. That said, the estrous cycle dog owners observe varies enormously between breeds and between individual dogs. A Basenji may cycle only once a year. A German Shepherd may cycle every 4 to 5 months. Some dogs split their heat into two separate episodes of bleeding. Others show almost no discharge at all and the owner notices only a swollen vulva and a change in behavior.

This article covers each stage in order, explains the hormones driving them, and explains why progesterone testing, not calendar counting or visual signs, is the only reliable way to time breeding.

This article is educational and is not a substitute for veterinary diagnosis or treatment.

The Four Stages of the Dog Heat Cycle at a Glance

Stage	Typical duration	Key signs	Dominant hormones	Breeding or spay decision
Proestrus	7 to 10 days	Vulval swelling, bloody discharge, males attracted but female refuses, restless behavior	Estrogen rising, progesterone still low	Too early to breed. Spay risk is elevated.
Estrus	5 to 9 days	Standing heat, flagging the tail, discharge lightens to straw color, female accepts the male	Estrogen peaks then falls, LH surge, progesterone rises steeply	Fertile window. Time breeding by progesterone, not by day count.
Diestrus	60 to 90 days	Discharge stops, vulva shrinks, possible nesting, appetite changes, mammary development	Progesterone high throughout, then declines at the end	Pregnancy or pseudopregnancy. Spay risk is elevated.
Anestrus	4 to 5 months	No vulval swelling, no discharge, no sexual behavior	All reproductive hormones low	Best window for elective spay.

The table is a framework, not a schedule. Every number in it shifts with breed, age, body size, season and individual variation.

Proestrus: The First Stage of the Canine Heat Cycle

Proestrus is the follicular buildup. Follicles in the ovary grow and secrete estrogen, which thickens the lining of the vagina and uterus and produces the signs owners recognize first.

What owners see during proestrus

The vulva swells and becomes turgid. A bloody or serosanguineous discharge appears, ranging from a few drops to a steady flow that soils bedding. Some dogs groom constantly and keep themselves clean, so the discharge may be easy to miss in a long-coated breed. Others leave obvious spots on the floor.

Behavior changes are common. Many bitches become restless, urinate more frequently, and scent-mark. Male dogs will follow them and attempt to mount, but the female refuses. She may snap, sit down, or move away when a male approaches. That refusal is the single most useful behavioral marker separating proestrus from estrus.

The hormonal pattern in proestrus

Estrogen climbs steadily through proestrus and reaches its highest concentration near the end of the stage. Progesterone stays low until the LH surge. During proestrus, the reproductive tract is under estrogen dominance, and the cervix is beginning to relax. A study using ultrasound elastography found that cervical tissue is softer during proestrus and the pre-ovulatory phase than during anestrus and diestrus, when the cervix is stiffer [3].

Estrogen also drives changes deeper in the tract. A study of ovarian tissue across the cycle found that the receptor CXCR4 peaks in interstitial ovarian cells during proestrus, and that FGFR3 expression in granulosa cells increases progressively from prepuberty toward estrus [4]. These are molecular markers of follicular growth rather than things an owner can observe, but they confirm that proestrus is an active, building phase rather than a passive waiting period.

The vaginal flora question

Owners often worry about discharge and bacteria. A large retrospective study of vaginal swabs from 696 healthy dogs in proestrus and estrus found that only 3.7 percent of samples were completely free of bacteria [5]. In other words, a positive culture from a normal dog in heat is expected and does not by itself mean infection. The authors of that study specifically framed their findings as a way to prevent the misuse of antibiotics in veterinary medicine [5]. Vaginal bacteria are normal in a cycling bitch.

Estrus: The Stage of Standing Heat

Estrus is the fertile stage. It is defined by receptivity: the bitch stands still for the male, holds her tail to one side (called flagging), and allows mounting and mating.

What owners see during estrus

The vulval swelling may soften slightly. The discharge often becomes lighter in color, shifting from bloody red to a pinkish or straw-colored tint, and may decrease in volume. Behavior is the giveaway. The dog becomes flirtatious, may mount other dogs or inanimate objects, and actively seeks out males. When a male mounts, she stands firm rather than sitting or pulling away.

The hormonal pattern in estrus

Estrogen peaks at or just before the start of estrus and then falls. The fall in estrogen releases the pituitary from negative feedback, and luteinizing hormone surges. The LH surge triggers ovulation, which in dogs occurs roughly 24 to 48 hours after the surge, and the oocytes are released in an immature form that requires about 2 additional days in the oviduct before they can be fertilized. This is why the fertile window in dogs sits in the early-to-mid part of estrus and often extends into early diestrus, not at the very start of bleeding.

Progesterone rises steeply during estrus. It is produced by the follicle walls as they transform into corpora lutea after ovulation. A clinical case study tracking a German Shepherd bitch during the periovulatory period recorded a progesterone concentration of 5.97 ng/mL at the point when the level suggested ovulation was approaching, and that value rose to 13.98 ng/mL within the following three days [6]. That trajectory, a rapid climb through the low single digits and into the teens, is the pattern veterinarians look for when timing breeding.

Why progesterone testing is the only reliable timing method

Calendar counting fails because proestrus and estrus overlap unpredictably between dogs. A bitch may bleed for 4 days before ovulation or for 14. Visual signs fail for the same reason: discharge color, vulval size and behavior all vary between individuals and even between cycles in the same dog.

Progesterone is different. It rises in a predictable pattern relative to ovulation, and serum measurement gives a number that can be interpreted against known thresholds. A study comparing clinical, endocrinological and histomorphological staging of the canine cycle found that dogs judged to be in clinical and endocrinological estrus had already entered the luteal phase according to tissue histology [7]. That mismatch is exactly why clinical signs alone are unreliable, and why the same study recommended comparing estradiol and progesterone concentrations when defining the transition from the follicular phase to ovulation [7].

The same study also warned that absolute progesterone concentrations can deviate depending on the assay used [7]. A value of 5 ng/mL on one analyzer is not guaranteed to read 5 ng/mL on another. Serial sampling on the same platform, interpreted by a veterinarian, is the practical answer.

The vaginal and uterine environment during estrus

The reproductive tract is not sterile at any stage, and the bacterial population shifts with the cycle. A study of uterine lavage samples from 20 adult mixed-breed bitches found the lowest number of isolated bacteria during estrus and the highest during diestrus and anestrus [8]. A separate study of vaginal microflora in 39 dogs identified Escherichia coli, Staphylococcus pseudintermedius and Streptococcus canis as the most common isolates, and found that the total bacterial count was almost the same in healthy and infected dogs and across cycle stages [9]. That study concluded that bacterial culturing of vaginal swabs from bitches without signs of genital disease is of little value [9].

The practical takeaway is that a culture result alone should not drive treatment decisions in a healthy dog in heat.

Diestrus: The Luteal Phase

Diestrus begins when the bitch stops accepting the male, usually 5 to 9 days after the start of estrus. It lasts about 60 to 90 days regardless of whether the dog conceived.

What owners see during diestrus

The discharge stops. The vulva shrinks back toward normal size. Behavior often settles, though some dogs become clingy, nest, carry toys, or show appetite changes. Mammary glands may enlarge and even produce milk in the later part of diestrus. This can happen whether or not the dog is pregnant, which is why the term pseudopregnancy exists.

The hormonal pattern in diestrus

Progesterone stays high for most of diestrus, maintained by the corpora lutea. A large study of 126 bitches measured progesterone during pregnancy and non-pregnant diestrus. At 11 to 19 days post-ovulation, mean concentrations were 30.23 ng/mL in pregnant dogs and 28.45 ng/mL in non-pregnant dogs. At 23 to 32 days post-ovulation, values were 22.73 ng/mL and 22.59 ng/mL respectively. Only at the final sampling, 52 to 60 days post-ovulation, did the groups diverge significantly, with pregnant dogs at 6.68 ng/mL and non-pregnant dogs at 3.17 ng/mL [10].

That overlap is the key point. Progesterone alone cannot tell you whether a dog is pregnant, because the non-pregnant luteal phase produces nearly identical concentrations for most of its course. Pregnancy diagnosis requires ultrasound or another imaging method.

The canine corpus luteum is the main source of reproductive steroids during diestrus, and it remains active even without pregnancy, a state described as physiological pseudopregnancy [11]. A study of estrogen receptor expression in the corpus luteum found that the ESR2 to ESR1 ratio increases around days 40 and 50 after ovulation and then declines during luteal regression [11].

Progesterone is essential for maintaining pregnancy. A study of French Bulldog bitches found that dogs with luteal insufficiency on days 18 to 22 of pregnancy had serum progesterone of 9.90 nmol/L compared with 55.60 nmol/L in controls [12]. That gap illustrates how much progesterone production matters for pregnancy maintenance, and why hypoluteoidism is a recognized cause of pregnancy loss.

The mammary gland during diestrus

The mammary gland changes measurably across the cycle. A study of six intact female beagles using B-mode and contrast-enhanced ultrasound found that all mammary glands increased in thickness during early and late diestrus and took on a more heterogeneous appearance [13]. A separate study of mammary tissue from 18 bitches found a significantly higher proliferation index during early diestrus across all mammary gland components, with the highest proliferation in epithelial cells within the end buds [14].

This matters clinically because mammary tumors are common in dogs, and the hormonal environment of diestrus drives proliferation in normal tissue. It is background physiology, not a diagnosis, but it explains why veterinarians pay attention to cycle stage when evaluating mammary changes.

Endometrial antioxidant balance

A study of endometrial tissue from 25 dogs across five cycle phases found that superoxide dismutase activity decreased from anestrus to estrus, while catalase activity increased from anestrus to diestrus. Glutathione-dependent enzymes stayed relatively stable. The only clear sign of oxidative stress related lipid peroxidation appeared between anestrus and proestrus [15]. The overall interpretation was that the normal canine endometrium maintains a delicate antioxidant balance rather than accumulating oxidative damage across the cycle [15].

Anestrus: The Resting Stage

Anestrus is the reproductive quiet period. It lasts about 4 to 5 months in most bitches and is the stage when the ovary is inactive and reproductive hormones are at baseline.

What owners see during anestrus

Nothing reproductive. The vulva is normal in size, there is no discharge, and the dog shows no sexual behavior. Coat shedding, appetite and energy are governed by other factors.

The hormonal pattern in anestrus

Estrogen, progesterone and LH are all low. The uterus and cervix return to a resting state. Ultrasound elastography shows the cervix is stiffer during anestrus than during proestrus and pre-ovulatory estrus [3]. Ovarian remodeling markers follow the same pattern: FGFR3 immunoreactivity in granulosa cells declines sharply in anestrus compared with estrus [4].

Anestrus is not a dead period biologically. A study of anestrus bitches given kisspeptin, a hormone that stimulates the reproductive axis, found that a 1 microgram per kilogram dose effectively stimulated LH and FSH release, though it did not reliably induce estrus [16]. That work confirms the pituitary and ovary remain responsive during anestrus even though the cycle is paused.

Anestrus is the safest window for elective spay

Anestrus is the preferred stage for elective ovariohysterectomy. The reproductive tract is small, blood supply is reduced, and hormone-driven tissue changes are absent. Spaying during estrus or diestrus is technically more demanding because the uterus and its vessels are engorged and the tissue is more friable.

How Long Does a Dog Stay in Heat?

A single heat, meaning proestrus plus estrus, typically lasts 12 to 21 days. The full cycle, including diestrus and anestrus, lasts about 5 to 8 months.

The phrase "how long is a dog's period" usually refers to the visible bleeding, which corresponds mostly to proestrus and the early part of estrus. That bleeding usually lasts 7 to 14 days, though it can be shorter or longer.

Owners should expect variation. Breed size is one factor. Small breeds often cycle more frequently than giant breeds. Age is another. The first heat usually occurs between 6 and 12 months, but can be delayed to 18 months or later in large breeds. Season can matter in some lines. Individual variation is the largest factor of all.

The Hormonal Sequence Behind the Heat Cycle

The diagram below traces the hormonal sequence that drives the four stages.

flowchart TD
    A[Anestrus] --> B[Estrogen rises]
    B --> C[Proestrus]
    C --> D[Estrogen peaks]
    D --> E[LH surge]
    E --> F[Ovulation]
    F --> G[Progesterone rises]
    G --> H[Estrus]
    H --> I[Diestrus]
    I --> J[Progesterone high]
    J --> K[Luteal regression]
    K --> A

The loop repeats roughly every 6 months in most bitches, with the exact interval set by breed and individual biology.

Clinical Relevance, Limitations and Common Mistakes

The most common mistake is counting days from the first spot of blood and breeding on a fixed schedule. Ovulation timing varies too much between dogs for that to work. Progesterone testing is the reliable method, and it usually requires two or more samples spaced a few days apart to catch the rise.

A second mistake is assuming that a dog cannot get pregnant on her first heat. She can. A third is assuming that a dog who refuses a male is not in season. She may be in proestrus, which is not fertile, and she may accept him days later.

A fourth mistake is treating a positive vaginal culture as an infection. Normal bitches in heat carry a rich vaginal flora, and most swabs grow bacteria [5][9]. Antibiotics are not indicated on culture results alone in a dog without clinical signs of genital disease.

A fifth mistake is scheduling an elective spay during estrus or diestrus without discussing timing with the veterinarian. Surgery during these stages carries higher risk because the reproductive tract is enlarged and vascular. Anestrus is the preferred window.

A sixth mistake is assuming that a non-pregnant dog cannot show signs of pregnancy. Pseudopregnancy is normal canine physiology, driven by the same corpus luteum that supports a real pregnancy [11]. Mammary development and nesting behavior can occur without any pregnancy at all.

A seventh mistake is relying on a single progesterone value. Assays differ, and absolute numbers can deviate between platforms [7]. Trends over serial samples are more informative than any single number.

Individual cases need a veterinarian. This article describes general patterns, not a diagnostic protocol for a specific dog.

Frequently Asked Questions

How long does a dog heat cycle last?

A full cycle lasts about 5 to 8 months. The visible heat, meaning proestrus plus estrus, lasts roughly 12 to 21 days.

How long does a dog bleed during her period?

Bleeding usually lasts 7 to 14 days, covering most of proestrus and the early part of estrus. Some dogs bleed longer, and some bleed very little.

Can a dog get pregnant on her first heat?

Yes. A dog can conceive on her first heat cycle if she ovulates and is bred during the fertile window.

What are the first signs of a dog going into heat?

Vulval swelling and a bloody discharge are usually first. Restlessness, frequent urination and increased attention from male dogs follow.

How do I know when my dog is in standing heat?

She stands still and holds her tail to the side when a male mounts. She may also flag, mount other dogs, or seek out males.

Why is progesterone testing better than counting days?

Progesterone rises in a predictable pattern relative to ovulation, while bleeding and behavior vary too much between dogs to be reliable. Serial progesterone samples give a number that can be interpreted against known thresholds.

Can a dog be spayed while in heat?

Yes, but the risk is higher because the reproductive tract is engorged and more vascular. Anestrus is the preferred window for elective spay.

How often do dogs go into heat?

Most dogs cycle every 5 to 8 months, averaging about every 6 months. Some breeds cycle only once a year, and some cycle every 4 months.