# Reproductive Hormone Testing in Mares ## Quick Answer - Reproductive hormone testing in mares supports cycle monitoring, pregnancy diagnosis, and placental function assessment through measurement of progesterone, estrogens, and related steroids. - Practical next step: combine transrectal ultrasonography with serial blood sampling for progesterone and estrogen to distinguish normal cyclicity from pregnancy failure or placental disease. - Important limitation: endocrine values vary with gestational stage, season, and individual mare, so single measurements without ultrasound context can mislead clinical decisions. ## Understanding the Equine Reproductive Endocrine Axis The mare's reproductive cycle depends on a coordinated sequence of hormonal events originating from the hypothalamus, pituitary gland, ovaries, and ultimately the fetoplacental unit during pregnancy. Gonadotropin-releasing hormone from the hypothalamus stimulates the pituitary to release luteinizing hormone and follicle-stimulating hormone, which drive follicular development and ovulation. The corpus luteum formed after ovulation produces progesterone, while developing follicles produce estrogens, primarily estradiol-17β. These steroid hormones feed back to regulate further gonadotropin release and establish the characteristic pattern of estrus and diestrus. Seasonal influences dominate equine reproductive function. Mares rarely ovulate during winter months, and the onset of cyclic activity is triggered by increasing day length. This photoperiodic control means that hormone testing during the transition period, the interval between winter anestrus and regular cyclicity, requires careful interpretation because follicular development may occur without ovulation and progesterone concentrations remain low. The [Merck Veterinary Manual](https://www.merckvetmanual.com/) provides background on normal equine reproductive physiology and seasonal breeding patterns that inform interpretation of endocrine results. Understanding the endocrine axis matters for practical decisions about breeding management. When a mare fails to show estrus, fails to conceive, or loses a pregnancy, hormone measurements help determine which component of the axis has failed. Low progesterone after ovulation suggests luteal insufficiency. Persistent high progesterone without pregnancy suggests prolonged diestrus or early embryonic loss. Rising estrogen in a pregnant mare suggests active fetoplacental function. Each pattern points toward different management interventions. ## Hormone Tests Available for Mares ### Progesterone Assays Progesterone measurement is the most frequently used endocrine test in equine reproductive practice. Progesterone is produced by the corpus luteum, and its concentration in blood reflects luteal function. During diestrus and early pregnancy, progesterone concentrations rise and remain elevated until luteolysis or the transition to placental progesterone production. The [Merck Veterinary Manual](https://www.merckvetmanual.com/) describes progesterone testing as a tool for confirming ovulation, monitoring luteal function, and diagnosing pregnancy failure. Blood sampling for progesterone requires attention to timing. A single sample taken during diestrus confirms that ovulation has occurred if progesterone is elevated. Serial sampling provides more information about luteal persistence or regression. In pregnant mares, progesterone concentrations typically remain elevated through the first trimester, then decline as the placenta assumes steroid production. The 2023 review in [Reproduction in Domestic Animals](https://pubmed.ncbi.nlm.nih.gov/37191550) notes that endocrine profiles including progesterone and its metabolite 5α-dihydroprogesterone can be evaluated simultaneously using liquid chromatography-tandem mass spectrometry, providing functional information about fetal and placental competence. ### Estrogen Assays Estrogens, particularly estradiol-17β, originate from developing follicles and later from the fetoplacental unit. During the estrous cycle, estradiol rises as the dominant follicle matures, peaks near ovulation, and declines afterward. Measuring estradiol helps confirm follicular activity and can indicate when ovulation is approaching, though ultrasound remains the standard method for tracking follicle size and ovulation timing. During pregnancy, estrogen patterns change dramatically. The 2020 [Theriogenology review on biomarkers for placental disease](https://pubmed.ncbi.nlm.nih.gov/32088026) reports that estradiol-17β concentrations, in both free and conjugated forms, are drastically reduced in plasma of mares with placentitis. This finding makes estrogen measurement valuable for assessing placental health in late gestation. Rising estrogen concentrations toward the end of pregnancy reflect fetal and placental maturation, while falling concentrations may signal placental compromise. ### Gonadotropin Measurements Luteinizing hormone and follicle-stimulating hormone can be measured to evaluate pituitary function and response to treatment. The 1993 study on [gonadotropin-releasing hormone agonist administration](https://pubmed.ncbi.nlm.nih.gov/8250401) in seasonally anestrous mares demonstrated that ovulation was accompanied by increased circulating concentrations of luteinizing hormone and decreased follicle-stimulating hormone values. These measurements help researchers and clinicians understand why some mares respond to hormonal manipulation while others do not. In practice, gonadotropin measurements are less commonly used than progesterone and estrogen assays because they require specialized laboratory capabilities and interpretation is complex. However, they become relevant when evaluating mares that fail to respond to standard ovulation induction protocols or when investigating suspected pituitary dysfunction. ### Progestogens and Other Steroids The term progestogens encompasses progesterone and related compounds with progestational activity. During pregnancy, the equine placenta produces a variety of progestogens that differ from ovarian progesterone. The 2023 review in [Reproduction in Domestic Animals](https://pubmed.ncbi.nlm.nih.gov/37191550) emphasizes that endocrine profiles including progesterone, 5α-dihydroprogesterone, other metabolites, androgens, and estrogens can be evaluated simultaneously using liquid chromatography-tandem mass spectrometry. This comprehensive approach provides more functional information than measuring a single hormone. The 2020 [Theriogenology review](https://pubmed.ncbi.nlm.nih.gov/32088026) notes that mares with chronic placentitis have an increase in peripheral progestogens, while acutely infected mares display reduced progestogen concentrations. This distinction matters clinically because it affects interpretation of serial hormone measurements in pregnant mares. A rising progestogen pattern may indicate chronic placental inflammation, while a sudden drop may signal acute infection or impending pregnancy loss. ## At a Glance: Hormone Tests and Their Clinical Use | Hormone | Source | Clinical Indication | Interpretation Caveat | |---------|-------|---------------------|----------------------| | Progesterone | Corpus luteum, placenta | Confirm ovulation, monitor luteal function, assess early pregnancy | Single samples cannot distinguish pregnancy from prolonged diestrus | | Estradiol-17β | Follicles, fetoplacental unit | Track follicular maturation, assess placental function | Concentrations vary with stage of cycle and gestation | | Progestogens | Placenta | Monitor late pregnancy, detect placental disease | Chronic placentitis raises levels while acute infection lowers them | | Luteinizing hormone | Pituitary | Evaluate response to ovulation induction | Requires serial sampling and specialized laboratory support | | Pregnant mare serum gonadotropin | Endometrial cups | Diagnose pregnancy between days 40 and 120 | Not produced after endometrial cup regression | ## Cycle Monitoring with Hormone Testing ### Confirming Ovulation Progesterone measurement provides a reliable method for confirming that ovulation has occurred. When a mare is bred and the practitioner needs to verify ovulation, a blood sample taken 5 to 7 days after breeding will show elevated progesterone if ovulation occurred. This information guides decisions about repeat breeding, administration of luteolytic agents, or initiation of pregnancy management protocols. The [Merck Veterinary Manual](https://www.merckvetmanual.com/) describes the corpus luteum as the primary source of progesterone during diestrus and early pregnancy. When progesterone remains low after breeding, the practitioner should consider that ovulation did not occur, that the mare is in transition from anestrus, or that sampling was performed too early. Serial sampling over several days provides more definitive information than a single measurement. ### Distinguishing Estrus from Diestrus Behavioral signs of estrus, including tail raising, urination, and acceptance of the stallion, are useful but not always reliable. Some mares show ambiguous behavior, particularly during the transition period or when they have uterine pathology. Progesterone measurement helps distinguish true estrus from diestrus because progesterone is low during estrus and elevated during diestrus. The 1977 [Equine Veterinary Journal review on advances in equine reproduction](https://pubmed.ncbi.nlm.nih.gov/13992) notes that spontaneous prolonged diestrus is common in summer but may be effectively treated. This condition, sometimes called persistent corpus luteum, occurs when the corpus luteum fails to regress at the expected time. Affected mares maintain elevated progesterone and do not return to estrus. Hormone testing identifies this pattern and supports treatment decisions. ### Managing the Transition Period The transition from winter anestrus to spring cyclicity presents particular challenges for hormone testing. During this period, mares may develop follicles that fail to ovulate, and progesterone remains low despite apparent follicular activity. Estrogen concentrations may rise as follicles grow, but without ovulation, progesterone does not increase. This pattern can confuse owners who observe estrus behavior and assume the mare is ready to breed. The 1993 study on [gonadotropin-releasing hormone agonist administration](https://pubmed.ncbi.nlm.nih.gov/8250401) in seasonally anestrous mares found that constant administration of the agonist induced ovulation in some mares, but the percentage of mares ovulating and the lack of reproducibility of effect indicated that this approach was inappropriate for reliable manipulation of breeding activity in commercial broodmares. This finding underscores the variability of the transition period and the need for careful monitoring instead of assumption that treatment will produce consistent results. ## Pregnancy Diagnosis and Monitoring ### Early Pregnancy Diagnosis Transrectal ultrasonography remains the standard method for early pregnancy diagnosis in mares, allowing detection of the embryonic vesicle as early as 10 to 12 days after ovulation. The 2023 review in [Reproduction in Domestic Animals](https://pubmed.ncbi.nlm.nih.gov/37191550) describes ultrasound as providing structural information on embryonic and fetal growth, while endocrine profiles provide functional information on fetal and placental competence. Combining both approaches gives the most complete picture of pregnancy health. Hormone testing complements ultrasound in specific situations. When ultrasound is not available, or when the practitioner needs additional information about placental function, progesterone and estrogen measurements add value. The 1977 [Equine Veterinary Journal review](https://pubmed.ncbi.nlm.nih.gov/13992) notes that pregnancy can be diagnosed efficiently by rectal examination and by immunological assay of pregnant mare serum gonadotropin. This hormone, produced by endometrial cups, appears in maternal blood between days 40 and 120 of pregnancy and provides a pregnancy test during that window. ### Assessing Fetal and Placental Health As pregnancy advances, endocrine monitoring becomes increasingly important for detecting placental disease. The 2020 [Theriogenology review on biomarkers for placental disease](https://pubmed.ncbi.nlm.nih.gov/32088026) reports that placentitis is an important cause of abortion, stillbirth, and neonatal death in horses. Diagnosis based on clinical signs, including premature mammary gland development and vulvar discharge, and ultrasonography of the caudal placental pole can miss early and subtle cases. The review describes several blood markers that change in association with placentitis. Mares with chronic placentitis have increased peripheral progestogens, while acutely infected mares display reduced progestogen concentrations. Estradiol-17β concentrations are drastically reduced in plasma of mares with placentitis. Acute-phase proteins, particularly serum amyloid A, increase in plasma of affected mares. Alpha-fetoprotein, expressed in the fetoplacental unit, also increases in plasma of mares with placentitis. These findings support serial hormone monitoring in mares at risk for placental disease, including those with a history of placentitis, premature lactation, or vulvar discharge. The 2023 review in [Reproduction in Domestic Animals](https://pubmed.ncbi.nlm.nih.gov/37191550) notes that endocrine information can be used in making clinical decisions including the need for progestin supplementation or when it can cease, and even estimating gestational stage in mares that cannot be easily palpated or scanned, as with mini-breeds or intractable animals. ### Monitoring Late Pregnancy Toward the end of pregnancy, hormone patterns shift as the fetoplacental unit matures. The 1977 [Equine Veterinary Journal review](https://pubmed.ncbi.nlm.nih.gov/13992) notes that plasma progesterone levels rise and estrogen levels fall toward the end of pregnancy. These changes reflect the transition from ovarian to placental steroid production and the maturation of the fetal adrenal axis. The 2023 review in [Reproduction in Domestic Animals](https://pubmed.ncbi.nlm.nih.gov/37191550) describes how combined ultrasound and hormonal analysis provides insight into fetoplacental well-being and the progress of pregnancy, helping to identify problems needing therapeutic intervention. Measurement of combined thickness of the uterus and placenta, visual assessment of fetal fluids, activity, heart rate, and multiple biometrics involving the fetal head, eyes, limbs, and joints provides structural information. Endocrine profiles provide functional information about whether the placenta is producing adequate steroids to maintain pregnancy. ## The Role of hCG Stimulation Testing Human chorionic gonadotropin is used in equine practice to induce ovulation in mares with a mature follicle. The hormone mimics luteinizing hormone and triggers the ovulatory cascade. While hCG stimulation is primarily a treatment instead of a diagnostic test, its response can provide information about ovarian function and follicular maturity. The 1993 study on [gonadotropin-releasing hormone agonist administration](https://pubmed.ncbi.nlm.nih.gov/8250401) provides context for understanding how the pituitary and ovary respond to hormonal stimulation. In mares that ovulated in response to treatment, ovulation was preceded by an increase in estradiol and luteinizing hormone concentrations. In mares that did not ovulate, luteinizing hormone increased during treatment while follicle-stimulating hormone either increased or did not change. This pattern suggests that the ovary, not the pituitary, limits the ovulatory response in some mares. Practitioners considering hCG stimulation should recognize that response varies among mares and across seasons. The 1993 study found considerable variation in ovulatory response with identical doses tested in consecutive or alternate years. This variability means that a mare that responds well one season may not respond the next, and repeated treatment does not guarantee consistent results. ## Practical Implementation: When to Test and What to Do ### Step 1: Define the Clinical Question Before ordering hormone tests, the practitioner should define the specific question to be answered. Is the goal to confirm ovulation, diagnose pregnancy, investigate failure to conceive, or monitor placental health in late gestation? Each question requires different sampling protocols and interpretation frameworks. For cycle monitoring, progesterone measurement 5 to 7 days after breeding confirms ovulation. For pregnancy diagnosis, ultrasound remains the primary tool, with hormone testing reserved for specific indications. For placental health, serial measurements of progestogens and estrogens provide the most useful information. ### Step 2: Select the Appropriate Test Progesterone assays are widely available and provide the most commonly needed information. Estrogen assays require more specialized laboratory support but add value in specific situations. Comprehensive steroid profiling using liquid chromatography-tandem mass spectrometry provides the most complete picture but is not available in all laboratories. The 2023 review in [Reproduction in Domestic Animals](https://pubmed.ncbi.nlm.nih.gov/37191550) describes how endocrine profiles including progesterone, 5α-dihydroprogesterone, other metabolites, androgens, and estrogens can be evaluated simultaneously using liquid chromatography-tandem mass spectrometry. This approach provides more functional information on fetal and placental competence and development than measuring a single hormone. ### Step 3: Interpret Results in Context Hormone concentrations must be interpreted in the context of the mare's reproductive stage, season, and clinical presentation. A progesterone concentration that indicates luteal activity in a cyclic mare may have different implications in a pregnant mare or a mare in transition from anestrus. The [Merck Veterinary Manual](https://www.merckvetmanual.com/) emphasizes that laboratory results should be interpreted in light of the complete clinical picture. Ultrasound findings, behavioral observations, and reproductive history all contribute to accurate interpretation. A single hormone measurement rarely provides sufficient information for a definitive diagnosis. ### Step 4: Document and Track Maintaining accurate records of hormone measurements, ultrasound findings, and clinical observations supports longitudinal monitoring and improves diagnostic accuracy. Serial measurements are more informative than single samples because they reveal trends that indicate improvement or deterioration. The [World Organisation for Animal Health](https://www.woah.org/en/what-we-do/animal-health-and-welfare) emphasizes the importance of animal health surveillance and reporting. For breeding operations, tracking reproductive performance and hormone data contributes to herd-level health management and early detection of problems. ## Records and Measurements Accurate record keeping is essential for effective use of hormone testing in equine practice. Each mare should have a reproductive record that includes: - Date of each hormone measurement and the specific test performed - Results with units and laboratory reference ranges - Ultrasound findings at the time of sampling - Stage of the estrous cycle or gestation - Breeding dates and outcomes - Treatments administered and responses observed - Clinical signs, including estrus behavior, vulvar discharge, or mammary development Serial measurements plotted over time reveal patterns that single measurements cannot. For example, a gradual decline in progestogens in a pregnant mare may indicate chronic placental insufficiency, while a sudden drop may signal acute infection. The 2020 [Theriogenology review](https://pubmed.ncbi.nlm.nih.gov/32088026) describes how mares with chronic placentitis have increased peripheral progestogens while acutely infected mares display reduced concentrations, emphasizing the importance of understanding the pattern of change. Laboratory records should include the assay method used, because different methods may produce different results for the same sample. The 2023 review in [Reproduction in Domestic Animals](https://pubmed.ncbi.nlm.nih.gov/37191550) notes that liquid chromatography-tandem mass spectrometry allows simultaneous evaluation of multiple steroids, providing more comprehensive information than immunoassays that measure a single hormone. ## Common Failure Patterns and Their Interpretation ### Prolonged Diestrus A mare that maintains elevated progesterone without pregnancy may have a persistent corpus luteum. The 1977 [Equine Veterinary Journal review](https://pubmed.ncbi.nlm.nih.gov/13992) notes that spontaneous prolonged diestrus is common in summer but may be effectively treated. This condition occurs when the corpus luteum fails to regress at the expected time, and affected mares do not return to estrus. Hormone testing identifies this pattern by showing elevated progesterone in a nonpregnant mare. Ultrasound confirms the absence of pregnancy and may reveal the persistent corpus luteum. Treatment typically involves administration of a luteolytic agent, but the specific protocol should be determined by the attending veterinarian. ### Early Embryonic Loss Early embryonic loss occurs when a pregnancy fails after initial diagnosis. The 1977 [Equine Veterinary Journal review](https://pubmed.ncbi.nlm.nih.gov/13992) notes that service at the foal heat is associated with an increased prevalence of early embryonic death. Hormone testing may reveal declining progesterone or progestogen concentrations that indicate luteal failure or placental dysfunction. The 2023 review in [Reproduction in Domestic Animals](https://pubmed.ncbi.nlm.nih.gov/37191550) describes how endocrine information can be used in making clinical decisions including the need for progestin supplementation or when it can cease. When early embryonic loss is suspected, serial hormone measurements help determine whether the pregnancy is progressing normally or failing. ### Placentitis Placentitis is an important cause of abortion, stillbirth, and neonatal death in horses. The 2020 [Theriogenology review](https://pubmed.ncbi.nlm.nih.gov/32088026) describes how diagnosis based on clinical signs and ultrasonography can miss early and subtle cases. Hormone testing provides objective means of diagnosing placentitis through single or serial measurements of blood markers. The review reports that mares with chronic placentitis have increased peripheral progestogens, while acutely infected mares display reduced progestogen concentrations. Estradiol-17β concentrations are drastically reduced in plasma of mares with placentitis. These patterns help distinguish chronic from acute disease and guide treatment decisions. ### Transition Period Anovulation During the transition from winter anestrus to spring cyclicity, mares may develop follicles that fail to ovulate. The 1993 study on [gonadotropin-releasing hormone agonist administration](https://pubmed.ncbi.nlm.nih.gov/8250401) found that constant administration of the agonist induced ovulation in some anestrous mares but not others, with considerable variation in response. This variability makes the transition period particularly challenging for breeding management. Hormone testing during the transition period reveals low progesterone despite follicular development, confirming that ovulation has not occurred. Estrogen concentrations may rise as follicles grow, but without ovulation, progesterone remains low. This pattern indicates that the mare is not yet ready to breed and that further monitoring or intervention is needed. ## Diagnostic Workflow for Hormone Testing The following workflow illustrates how a practitioner can integrate hormone testing with ultrasound and clinical findings to reach a management decision. This sequence applies to a mare presented for cycle monitoring or pregnancy evaluation. ```mermaid flowchart TD A[Mare presented for breeding or pregnancy evaluation] --> B{Clinical question} B -->|Confirm ovulation| C[Progesterone assay 5 to 7 days after breeding] B -->|Diagnose pregnancy| D[Ultrasound at 10 to 14 days] B -->|Monitor placental health| E[Serial progestogen and estrogen assays] C --> F{Progesterone elevated?} F -->|Yes| G[Ovulation confirmed, proceed with management] F -->|No| H[Consider anovulation, transition period, or early sampling] D --> I{Pregnancy detected?} I -->|Yes| J[Continue monitoring with ultrasound and endocrine profiles] I -->|No| K[Consider prolonged diestrus or early embryonic loss] E --> L{Progestogen pattern} L -->|Rising| M[Possible chronic placentitis, escalate to specialist] L -->|Falling| N[Possible acute infection or pregnancy loss, escalate urgently] ``` This workflow emphasizes that hormone testing works best when paired with structural information from ultrasound. The 2023 review in [Reproduction in Domestic Animals](https://pubmed.ncbi.nlm.nih.gov/37191550) describes how these two approaches provide complementary information about fetal and placental well-being. ## Welfare and Safety Considerations Hormone testing procedures are minimally invasive and generally well tolerated by mares. Blood sampling requires restraint and venipuncture, which carry small risks of hematoma, infection, or injury to the handler. The [American Veterinary Medical Association](https://www.avma.org/resources-tools/pet-owners) provides general guidance on responsible pet ownership and veterinary care that applies to equine patients as well. The [World Organisation for Animal Health](https://www.woah.org/en/what-we-do/animal-health-and-welfare) emphasizes the importance of animal health and welfare in veterinary practice. When performing hormone testing, practitioners should ensure that handling and sampling procedures minimize stress and discomfort. Proper restraint, clean technique, and appropriate needle selection reduce the risk of complications. Anabolic steroid use in horses warrants specific attention. The 1993 [Veterinary Clinics of North America review on anabolic steroids](https://pubmed.ncbi.nlm.nih.gov/8299016) notes that these compounds have been employed extensively in equine practice, but their usefulness is largely dependent on subjective opinions because only minimal studies have been carried out in horses. The review notes that reproductive efficiency in both stallions and mares may be temporarily impaired by anabolic steroid use, but this is quite rapidly reversible following cessation of treatment. Practitioners should be aware that anabolic steroids, including testosterone, are prohibited in most racing jurisdictions. The review notes that the presence of these substances in postrace urine samples constitutes use of an illegal substance. Sensitive ELISA tests for screening and gas chromatography-mass spectrometry for confirmation have been developed to detect their use. ## Limitations of Hormone Testing Hormone testing provides valuable information but has important limitations that practitioners must recognize. Single measurements often cannot distinguish between conditions that produce similar hormone patterns. For example, elevated progesterone in a nonpregnant mare could indicate prolonged diestrus, early embryonic loss, or a persistent corpus luteum. Ultrasound and clinical history are needed to differentiate these possibilities. The 2023 review in [Reproduction in Domestic Animals](https://pubmed.ncbi.nlm.nih.gov/37191550) emphasizes that endocrine profiles provide functional information while ultrasound provides structural information. Neither approach alone provides a complete picture. Combining both approaches provides unusual insight into fetoplacental well-being and the progress of pregnancy. Seasonal variation affects hormone concentrations and interpretation. The 1977 [Equine Veterinary Journal review](https://pubmed.ncbi.nlm.nih.gov/13992) notes that mares rarely ovulate in winter and that ovulation is induced by increase in daylight length. Hormone patterns during the transition period differ from those during regular cyclicity, and reference ranges may not apply across seasons. Laboratory variation also affects results. Different assay methods may produce different values for the same sample, and reference ranges vary among laboratories. Practitioners should use the same laboratory for serial measurements and interpret results using that laboratory's reference ranges. ## Professional Escalation Criteria Practitioners should involve a veterinary reproduction specialist when hormone testing reveals patterns that exceed their comfort level or when management is not progressing as expected. Specific situations that warrant specialist referral include: - Repeated early embryonic loss despite apparently normal hormone patterns - Placentitis suspected but not confirmed by ultrasound or hormone testing - Mares that fail to respond to standard ovulation induction protocols - Pregnancies with abnormal hormone patterns that suggest placental insufficiency - Cases requiring comprehensive steroid profiling using liquid chromatography-tandem mass spectrometry The [Cornell University College of Veterinary Medicine](https://www.vet.cornell.edu/) provides advanced diagnostic and treatment services for equine reproduction, including specialized hormone testing and referral consultation. Practitioners should establish relationships with referral centers before emergencies arise. The [Merck Veterinary Manual](https://www.merckvetmanual.com/) provides authoritative background on equine reproductive disorders and their diagnosis, supporting practitioner education and client communication. The [American Veterinary Medical Association](https://www.avma.org/resources-tools/pet-owners) offers resources for pet owners that help explain the value of veterinary care and diagnostic testing. ## Comparative Interpretation Table The following table summarizes how different hormone patterns guide clinical interpretation in common reproductive scenarios. This comparison supports decision making when a practitioner reviews laboratory results. | Clinical Scenario | Progesterone Pattern | Estrogen Pattern | Likely Interpretation | Recommended Action | |-------------------|---------------------|------------------|----------------------|-------------------| | Bred mare, day 7 after ovulation | Elevated | Low to moderate | Ovulation confirmed | Proceed with pregnancy management | | Bred mare, day 7 after ovulation | Low | Variable | Anovulation or early sampling | Repeat sampling, consider ultrasound | | Nonpregnant mare, prolonged elevated progesterone | Elevated | Low | Prolonged diestrus or persistent corpus luteum | Ultrasound to confirm, consider luteolytic treatment | | Pregnant mare, late gestation | Rising progestogens | Reduced estradiol | Possible chronic placentitis | Escalate to specialist, consider treatment | | Pregnant mare, late gestation | Falling progestogens | Reduced estradiol | Possible acute infection or impending loss | Urgent escalation to specialist | The 2020 [Theriogenology review](https://pubmed.ncbi.nlm.nih.gov/32088026) supports the distinction between chronic and acute placentitis patterns, while the 2023 review in [Reproduction in Domestic Animals](https://pubmed.ncbi.nlm.nih.gov/37191550) supports the value of combined structural and functional monitoring. ## Building a Mare-Specific Hormone Baseline for Serial Comparison Single hormone measurements provide limited information because normal values vary with breed, age, parity, season, and gestational stage. A mare with a progesterone concentration of 4 ng/mL might be in normal diestrus, while another mare with the same value might have a failing pregnancy. The 2023 review in [Reproduction in Domestic Animals](https://pubmed.ncbi.nlm.nih.gov/37191550) emphasizes that endocrine profiles provide functional information that gains meaning when interpreted alongside structural data from ultrasound. The most reliable way to interpret hormone values is to establish a mare-specific baseline through serial sampling during known reproductive states, then compare subsequent measurements against that individual's established pattern. ### Why Population Reference Ranges Fall Short Most commercial laboratories report hormone results against population-based reference ranges derived from groups of mares at similar reproductive stages. These ranges have utility for gross classification, such as distinguishing low from high progesterone, but they fail to capture the substantial individual variation that exists among normal mares. A mare that consistently maintains progesterone concentrations at the low end of the normal range during diestrus may have perfectly adequate luteal function for her own reproductive system. Conversely, a mare whose progesterone drops from her personal baseline of 8 ng/mL to 4 ng/mL may be experiencing luteal insufficiency even though 4 ng/mL falls within the population reference range. The 2020 [Theriogenology review on biomarkers for placental disease](https://pubmed.ncbi.nlm.nih.gov/32088026) illustrates this principle for placentitis. Mares with chronic placentitis show increased peripheral progestogens, while acutely infected mares display reduced concentrations. Without knowing the individual mare's baseline pattern, a single progestogen measurement cannot distinguish a normal value from an abnormal one. Serial sampling that reveals a trend away from the mare's established baseline provides far more diagnostic power than any single measurement compared to a population range. ### Establishing the Baseline During the Estrous Cycle The foundation of mare-specific hormone interpretation is a complete cycle profile. For a nonpregnant mare being evaluated for breeding soundness or recurrent cycle abnormalities, the practitioner should collect blood samples at defined points across at least one full estrous cycle. The [Merck Veterinary Manual](https://www.merckvetmanual.com/) provides background on normal equine estrous cycle length and hormonal patterns that inform this sampling protocol. A practical sampling schedule includes a sample during estrus when progesterone should be low, a sample 5 to 7 days after ovulation when progesterone should be elevated, and a sample 14 to 16 days after ovulation when progesterone should decline if luteolysis has occurred. This three-point profile establishes the mare's personal progesterone range for estrus, early diestrus, and late diestrus. The 1977 [Equine Veterinary Journal review on advances in equine reproduction](https://pubmed.ncbi.nlm.nih.gov/13992) notes that spontaneous prolonged diestrus is common in summer, so a mare that maintains elevated progesterone beyond the expected luteal lifespan should be identified through this baseline comparison. For estradiol, samples collected during follicular development and near ovulation establish the mare's peak estrogen range. The 1993 study on [gonadotropin-releasing hormone agonist administration](https://pubmed.ncbi.nlm.nih.gov/8250401) found that ovulation was preceded by an increase in estradiol and luteinizing hormone concentrations, confirming that estrogen rises predictably before ovulation in responding mares. Knowing the mare's typical preovulatory estradiol peak helps the practitioner recognize when follicular development is inadequate or when the mare is not approaching ovulation despite apparent follicle growth on ultrasound. ### Building the Pregnancy Baseline Once a mare is confirmed pregnant, the practitioner should establish a pregnancy-specific hormone baseline through serial sampling at defined gestational intervals. The 2023 review in [Reproduction in Domestic Animals](https://pubmed.ncbi.nlm.nih.gov/37191550) describes how endocrine profiles including progesterone, 5α-dihydroprogesterone, other metabolites, androgens, and estrogens can be evaluated simultaneously using liquid chromatography-tandem mass spectrometry. This comprehensive approach provides more functional information than measuring a single hormone and supports the development of a detailed pregnancy baseline. A practical pregnancy monitoring schedule begins with a sample at pregnancy diagnosis, typically 14 to 16 days after ovulation, followed by samples at 30, 45, 60, 90, and 120 days of gestation. The 1977 [Equine Veterinary Journal review](https://pubmed.ncbi.nlm.nih.gov/13992) notes that during pregnancy, corpora lutea accumulate and all regress together at mid pregnancy, with plasma progesterone levels rising and estrogen levels falling toward the end of pregnancy. Understanding these expected changes allows the practitioner to recognize when a mare deviates from her established pattern. For mares with a history of pregnancy loss or placental disease, more frequent sampling is warranted. The 2020 [Theriogenology review](https://pubmed.ncbi.nlm.nih.gov/32088026) reports that placentitis can be missed when diagnosis relies only on clinical signs and ultrasonography of the caudal placental pole. Serial hormone measurements that reveal a trend away from the mare's established pregnancy baseline can identify early and subtle cases that would otherwise escape detection. ### Using the Baseline for Decision Making Once a mare-specific baseline is established, subsequent hormone measurements are interpreted by comparison to that baseline instead of to population reference ranges. This approach transforms hormone testing from a binary classification tool into a trend-monitoring system that detects subtle changes before they become clinically apparent. For example, a pregnant mare whose progestogen concentrations have been stable across multiple samplings but then show a 30 percent decline from her personal baseline warrants investigation even if the absolute value remains within the population reference range. The 2020 [Theriogenology review](https://pubmed.ncbi.nlm.nih.gov/32088026) notes that mares acutely infected with placentitis display reduced peripheral progestogen concentrations, so a downward trend from baseline may signal the onset of acute infection. Conversely, a mare whose progestogens rise above her established baseline may be developing chronic placentitis, which the review describes as associated with increased peripheral progestogens. The same principle applies to estradiol monitoring. The 2020 [Theriogenology review](https://pubmed.ncbi.nlm.nih.gov/32088026) reports that estradiol-17β concentrations are drastically reduced in plasma of mares with placentitis. A pregnant mare whose estradiol falls below her personal baseline range should be evaluated for placental compromise even if the absolute value does not fall below the laboratory's population reference range. ### Recording the Baseline for Longitudinal Comparison The mare-specific baseline has lasting value only if it is recorded and maintained in a format that supports longitudinal comparison. Each mare's reproductive record should include a dedicated hormone baseline section that documents the sampling dates, assay methods, results, and the reproductive state at the time of each sample. The [World Organisation for Animal Health](https://www.woah.org/en/what-we-do/animal-health-and-welfare) emphasizes the importance of animal health surveillance and reporting, and individual mare records contribute to this broader goal. The record should note the laboratory and assay method used for each measurement because different methods may produce different values for the same sample. The 2023 review in [Reproduction in Domestic Animals](https://pubmed.ncbi.nlm.nih.gov/37191550) notes that liquid chromatography-tandem mass spectrometry allows simultaneous evaluation of multiple steroids, providing more comprehensive information than immunoassays that measure a single hormone. If the practitioner changes laboratories or assay methods, the baseline should be reestablished with the new method before interpreting subsequent results. A practical record format includes a table with columns for sampling date, gestational or cycle day, hormone measured, result with units, assay method, laboratory, and clinical notes. This format allows the practitioner to scan the record quickly and identify trends. For mares monitored across multiple pregnancies, separate baseline sections for each pregnancy allow comparison of current pregnancy hormone patterns with those from previous successful or failed pregnancies. ### Troubleshooting When the Baseline Is Missing Practitioners frequently encounter mares without an established hormone baseline, particularly when managing a new patient or responding to an emergency. In these situations, the practitioner should collect a thorough reproductive history and begin baseline establishment immediately while also addressing the immediate clinical question. For a mare presented with suspected pregnancy loss, the practitioner should collect a blood sample immediately and then repeat sampling at 48 to 72 hour intervals to establish the trend. The 2023 review in [Reproduction in Domestic Animals](https://pubmed.ncbi.nlm.nih.gov/37191550) describes how endocrine information can be used in making clinical decisions including the need for progestin supplementation or when it can cease. Even without a prior baseline, the direction and rate of change between serial samples provides useful information about whether the pregnancy is progressing or failing. For a mare presented for breeding soundness evaluation, the practitioner should initiate a full cycle baseline protocol as described above. The [American Veterinary Medical Association](https://www.avma.org/resources-tools/pet-owners) provides general guidance on veterinary care that supports the value of thorough diagnostic evaluation. Establishing a baseline during the current cycle provides immediate information while also creating the foundation for future comparisons. ### Integrating the Baseline with Ultrasound Findings The mare-specific hormone baseline gains additional power when integrated with ultrasound findings. The 2023 review in [Reproduction in Domestic Animals](https://pubmed.ncbi.nlm.nih.gov/37191550) describes how ultrasound provides structural information on embryonic and fetal growth using parameters such as combined thickness of the uterus and placenta, visual assessment of fetal fluids, activity, heart rate, and multiple biometrics. When a hormone measurement deviates from the mare's baseline, ultrasound can determine whether structural changes accompany the hormonal change. For example, a pregnant mare whose progestogens decline from her personal baseline should undergo ultrasound evaluation of the fetal heart rate, fetal fluids, and placental thickness. The 2023 review notes that combined ultrasound and hormonal analysis provides unusual insight into fetoplacental well-being and the progress of pregnancy, helping to identify problems needing therapeutic intervention. A hormonal deviation without ultrasound changes may warrant closer monitoring, while a hormonal deviation accompanied by structural changes indicates a more advanced problem requiring immediate intervention. The 2020 [Theriogenology review](https://pubmed.ncbi.nlm.nih.gov/32088026) describes how diagnosis of placentitis is based on clinical signs and ultrasonography of the caudal placental pole, but early and subtle cases can be missed. The mare-specific hormone baseline provides an early warning system that prompts ultrasound evaluation before clinical signs develop. This proactive approach supports earlier detection and intervention, potentially improving outcomes for mares at risk of placental disease. ### Common Errors in Baseline Establishment Several common errors undermine the value of mare-specific hormone baselines. The most frequent error is inconsistent sampling timing. Hormone concentrations change rapidly during the estrous cycle and across gestation, so samples collected at different cycle or gestational stages cannot be compared directly. The practitioner should record the cycle day or gestational day for every sample and compare results only within the same reproductive stage. Another common error is changing laboratories or assay methods without reestablishing the baseline. Different assays may produce systematically different results for the same sample, so a value that appears to deviate from baseline may simply reflect a change in laboratory methodology. The practitioner should use the same laboratory and assay method for all serial comparisons or reestablish the baseline when methods change. A third error is failing to record clinical context alongside hormone values. A mare that is sick, stressed, or receiving medications may have hormone values that deviate from baseline for reasons unrelated to reproductive function. The 1993 [Veterinary Clinics of North America review on anabolic steroids](https://pubmed.ncbi.nlm.nih.gov/8299016) notes that reproductive efficiency in both stallions and mares may be temporarily impaired by anabolic steroid use, and this impairment is reflected in hormone patterns. Recording treatments and clinical conditions alongside hormone values supports accurate interpretation. ### When the Baseline Reveals a Problem When serial hormone measurements reveal a trend away from the mare's established baseline, the practitioner should follow a structured response protocol. First, repeat the measurement to confirm the deviation is real and not a laboratory error. Second, perform a thorough ultrasound examination to assess structural changes. Third, review the mare's clinical history for recent events that might explain the change, such as illness, stress, or medication administration. If the deviation is confirmed and no benign explanation is found, the practitioner should escalate to a veterinary reproduction specialist. The [Cornell University College of Veterinary Medicine](https://www.vet.cornell.edu/) provides advanced diagnostic and treatment services for equine reproduction, including specialized hormone testing and referral consultation. The [Merck Veterinary Manual](https://www.merckvetmanual.com/) provides authoritative background on equine reproductive disorders that supports practitioner education and client communication. The 2020 [Theriogenology review](https://pubmed.ncbi.nlm.nih.gov/32088026) describes how mares with chronic placentitis have increased peripheral progestogens while acutely infected mares display reduced concentrations. A mare whose progestogens rise above her baseline may have chronic placentitis requiring treatment, while a mare whose progestogens fall below her baseline may have acute infection requiring urgent intervention. The direction of deviation from baseline guides the urgency and nature of the response. ### Practical Baseline Establishment Protocol The following protocol provides a practical framework for establishing a mare-specific hormone baseline in a breeding practice. This protocol applies to mares presented for breeding soundness evaluation, mares entering a breeding program, and mares with a history of reproductive problems. For a nonpregnant mare, collect blood samples at three points during the estrous cycle: during estrus when progesterone should be low, 5 to 7 days after ovulation when progesterone should be elevated, and 14 to 16 days after ovulation when progesterone should decline if luteolysis has occurred. Record the results and the cycle day for each sample. This three-point profile establishes the mare's personal progesterone range for estrus, early diestrus, and late diestrus. For a pregnant mare, collect blood samples at pregnancy diagnosis and then at 30, 45, 60, 90, and 120 days of gestation. Record the results and the gestational day for each sample. This schedule establishes the mare's pregnancy hormone profile and provides the baseline for detecting deviations that may indicate placental disease or impending pregnancy loss. For mares at high risk of placental disease, including those with a history of placentitis, premature lactation, or vulvar discharge, increase sampling frequency to every 14 days from day 120 of gestation through term. The 2020 [Theriogenology review](https://pubmed.ncbi.nlm.nih.gov/32088026) describes how single or serial measurements of blood markers provide objective means of diagnosing placentitis. More frequent sampling in high-risk mares supports earlier detection of hormonal deviations from baseline. ### Limitations of the Baseline Approach The mare-specific baseline approach has limitations that practitioners must recognize. Baselines change over time as mares age and experience different reproductive events. A baseline established in a maiden mare may not apply to the same mare after multiple pregnancies. The practitioner should update the baseline periodically, particularly after significant reproductive events such as pregnancy, abortion, or treatment for reproductive disease. Baselines also vary across seasons. The 1977 [Equine Veterinary Journal review](https://pubmed.ncbi.nlm.nih.gov/13992) notes that mares rarely ovulate in winter and that ovulation is induced by increase in daylight length. Hormone patterns during the transition period differ from those during regular cyclicity, so a baseline established during the breeding season may not apply during the transition period. The practitioner should establish separate baselines for different reproductive states and seasons. Finally, the baseline approach requires discipline and record keeping. The [World Organisation for Animal Health](https://www.woah.org/en/what-we-do/animal-health-and-welfare) emphasizes the importance of animal health surveillance and reporting, and individual mare records contribute to this broader goal. Practices that maintain accurate, complete records will derive the most value from the mare-specific baseline approach, while practices that sample sporadically and record results incompletely will find the approach less useful. ## Frequently Asked Questions ### When should progesterone be measured in a breeding mare? Progesterone should be measured 5 to 7 days after breeding to confirm that ovulation occurred. A single elevated sample confirms luteal activity, while serial samples provide information about luteal persistence. The [Merck Veterinary Manual](https://www.merckvetmanual.com/) describes progesterone testing as a tool for confirming ovulation and monitoring luteal function. ### Can hormone testing replace ultrasound for pregnancy diagnosis? Hormone testing cannot replace ultrasound for early pregnancy diagnosis. Ultrasound detects the embryonic vesicle as early as 10 to 12 days after ovulation and provides structural information about fetal development. The 2023 review in [Reproduction in Domestic Animals](https://pubmed.ncbi.nlm.nih.gov/37191550) describes how ultrasound and endocrine testing provide complementary structural and functional information. ### What does a low progesterone level mean in a pregnant mare? Low progesterone in a pregnant mare may indicate luteal insufficiency, impending pregnancy loss, or sampling at a stage when progesterone has declined as the placenta assumes steroid production. The 2023 review in [Reproduction in Domestic Animals](https://pubmed.ncbi.nlm.nih.gov/37191550) notes that endocrine information can guide decisions about progestin supplementation and when it can cease. ### How is placentitis diagnosed with hormone testing? Placentitis is diagnosed by combining clinical signs, ultrasound findings, and hormone measurements. The 2020 [Theriogenology review](https://pubmed.ncbi.nlm.nih.gov/32088026) reports that mares with chronic placentitis have increased peripheral progestogens, while acutely infected mares display reduced progestogen concentrations. Estradiol-17β concentrations are drastically reduced in mares with placentitis. ### What is pregnant mare serum gonadotropin and when is it measured? Pregnant mare serum gonadotropin is produced by endometrial cups and appears in maternal blood between days 40 and 120 of pregnancy. The 1977 [Equine Veterinary Journal review](https://pubmed.ncbi.nlm.nih.gov/13992) describes immunological assay of this hormone as an efficient method for pregnancy diagnosis during that window. ### Why do hormone patterns vary during the transition period? The transition from winter anestrus to spring cyclicity involves gradual resumption of ovarian activity. Mares may develop follicles that fail to ovulate, resulting in low progesterone despite follicular development. The 1993 study on [gonadotropin-releasing hormone agonist administration](https://pubmed.ncbi.nlm.nih.gov/8250401) found considerable variation in ovulatory response, reflecting the unpredictable nature of the transition period. ### Are anabolic steroids safe for reproductive function in mares? Anabolic steroids may temporarily impair reproductive efficiency in mares, but this is quite rapidly reversible following cessation of treatment. The 1993 [Veterinary Clinics of North America review](https://pubmed.ncbi.nlm.nih.gov/8299016) notes that high doses may cause horses to become difficult to manage due to increased aggressiveness and other stallion-like behavior. ### When should a reproduction specialist be consulted? A reproduction specialist should be consulted when hormone testing reveals patterns that are difficult to interpret, when mares fail to respond to standard protocols, or when pregnancy loss recurs despite apparently normal management. Referral centers such as [Cornell University College of Veterinary Medicine](https://www.vet.cornell.edu/) provide advanced diagnostic services including comprehensive steroid profiling. ## Related Veterinary Guides - [Equine Reproductive Disorders: Diagnosis and Management](/knowledge/veterinary-medicine/equine-care/equine-reproductive-disorders-diagnosis-management) - [Equine Pregnancy Diagnosis: Methods and Clinical Application](/knowledge/veterinary-medicine/theriogenology/equine-pregnancy-diagnosis-methods-and-clinical-application) - [Cortisol Testing for Cushings and Addisons: ACTH Stimulation vs LDDST Tests](/knowledge/veterinary-medicine/at-home-diagnostics/cortisol-testing-for-cushings-and-addisons-acth-stimulation-vs-lddst-tests) - [Equine Recurrent Uveitis: Diagnosis, Long-Term Monitoring, and Eye Preservation](/knowledge/veterinary-medicine/equine-care/equine-recurrent-uveitis-diagnosis-long-term-monitoring-eye-preservation) - [Serial Fecal Testing for Monitoring Parasite Control Programs](/knowledge/veterinary-medicine/clinical-pathology/serial-fecal-testing-monitoring-parasite-control-programs) ## References and Further Reading - [Pet Care](https://www.avma.org/resources-tools/pet-owners). American Veterinary Medical Association. - [AAHA Guidelines](https://www.aaha.org/resources). American Animal Hospital Association. - [Global Guidelines](https://wsava.org/global-guidelines). World Small Animal Veterinary Association. - [Merck Veterinary Manual](https://www.merckvetmanual.com/). Merck Veterinary Manual. - [Cornell University College of Veterinary Medicine](https://www.vet.cornell.edu/). Cornell University. - [Animal Health and Welfare](https://www.woah.org/en/what-we-do/animal-health-and-welfare). World Organisation for Animal Health. - [Anabolic steroids.](https://pubmed.ncbi.nlm.nih.gov/8299016). The Veterinary clinics of North America. Equine practice, 1993. - [Recent advances in equine reproduction.](https://pubmed.ncbi.nlm.nih.gov/13992). Equine veterinary journal, 1977. - [Biomarkers for placental disease in mares.](https://pubmed.ncbi.nlm.nih.gov/32088026). Theriogenology, 2020. - [Pregnancy monitoring in mares: Ultrasonographic and endocrine approaches.](https://pubmed.ncbi.nlm.nih.gov/37191550). Reproduction in domestic animals = Zuchthygiene, 2023. - [Effect of constant administration of a gonadotropin-releasing hormone agonist on reproductive activity in mares: induction of ovulation during seasonal anestrus.](https://pubmed.ncbi.nlm.nih.gov/8250401). American journal of veterinary research, 1993. > This article is educational and is not a substitute for veterinary diagnosis or treatment. Contact a veterinarian for advice about an individual animal.