The Breeding Soundness Exam (BSE) is the main tool veterinarians use to assess a mare’s reproductive health. This comprehensive evaluation helps determine the mare’s suitability for breeding to improves the success of breeding programs. [2][3]

The primary purpose of the BSE is to identify and correct any potential reproductive issues or anatomical abnormalities that might interfere with successful conception, maintenance of pregnancy (gestation), or the birthing process (parturition) in mares.

The BSE is key to identifying reproductive problems early, allowing for timely intervention and management. This increases the likelihood of a successful breeding and healthy pregnancy for both the mare and developing foal. [1][2][4]

Through this examination, horse owners and breeders can make informed decisions about breeding management, optimize reproductive efficiency, and contribute to the long-term sustainability of equine breeding practices. In this article, we’ll explore the steps involved in a routine breeding sounds exam in the mare, as well as possible findings.

Breeding Soundness Exams for Mares

Despite ongoing advances in equine husbandry and veterinary care, achieving a full-term pregnancy in mares can be challenging. The foaling success rate is often measured below 60%, underscoring the complexities involved in equine reproduction. [1]

Breeding soundness exams (BSEs) are important evaluations performed on both stallions and mares to assess their potential for successful breeding. This process is essential for maintaining and improving herd genetics, animal health, and the overall efficiency of breeding programs.

A BSE in the mare will typically include a general health assessment and examination of the reproductive tract. More in-depth evaluations may include an analysis of hormone levels and uterine culture or biopsy.

Benefits of a Breeding Soundness Exam

The main goal of the BSE is to assess the reproductive fitness of the mare and detect health issues that might affect her ability to conceive, maintain pregnancy, or give birth to a healthy foal. Additional reasons to perform a breeding soundness exam (BSE) in a mare include:

  1. Assessment of Reproductive Health: The BSE helps evaluate overall reproductive health, including the examination of the reproductive tract and assessment of the estrous cycle. This provides valuable insights into the likelihood of successful conception and pregnancy.
  2. Evaluation of Fertility: The BSE assesses the mare’s fertility status by examining factors such as hormone levels, uterine health, and ovarian function.
  3. Detection of Anatomical Abnormalities: The BSE helps identify any structural or anatomical abnormalities that might affect the mare’s ability to conceive, carry a pregnancy to term, or give birth naturally.
  4. Optimization of Breeding Management: It assists owners in making informed decisions about breeding management, including selecting suitable breeding methods and timing.
  5. Maximization of Reproductive Efficiency: By addressing any potential reproductive challenges early on, the BSE helps maximize the reproductive efficiency and minimizes potential financial losses associated with unsuccessful breeding attempts.
  6. Prevention of Disease Transmission: By screening for infectious diseases and genetic disorders, the BSE helps prevent the spread of sexually-transmitted diseases and ensures the production of healthy offspring.

When to Schedule a Breeding Soundness Exam?

It is recommended to schedule a BSE at the beginning of the breeding season before selecting a stallion or initiating any breeding protocols. This will ensure that a mare is healthy enough to undergo pregnancy and parturition, and allow you to address issues that could interfere with breeding.

However, if a mare has a history of successful conception, gestation, and parturition, she may not require an annual BSE unless she encounters difficulties during the breeding season. [2]

Other scenarios for which your veterinarian may recommend a BSE include:

  • Maiden Mares: First-time mothers (also called maidens) benefit from the general reproductive health assessment that a BSE offers. It is particularly recommended for maidens over 12 years old, as age-related changes to the cervix can negatively impact breeding and foaling. [4]
  • Aging Mares: Generally, a mare’s fertility begins to decline between 10 and 15 years of age, increasing the risk of pregnancy loss. [8] Senior mares have a higher risk of reduced fertility and age-related endometritis. [9]
  • Assisted Reproductive Technologies: Before investing time and resources on assisted reproductive techniques, it’s advisable to conduct a BSE to assess the mare’s suitability for these procedures. Techniques include artificial insemination, deep horn insemination, and embryo transfer.
  • Breeding Failure or Pregnancy Loss: If a mare experiences breeding failure and/or pregnancy loss, a BSE can help identify underlying problems. Examples include subclinical endometritis, fibrosis, or anatomical defects. [4]
  • Pre-Purchase Examination: For those considering the purchase of a broodmare, conducting a BSE as part of a pre-purchase exam is crucial. It ensures that the mare has the capacity to carry a foal to term, providing valuable information for prospective buyers. [4]
  • Postpartum Evaluation: Some owners opt for a BSE after a mare gives birth to assess the condition of her reproductive tract and check for any fluid or infection prior to rebreeding. [4]

Reproductive Anatomy and Physiology of Mares

Understanding a mare’s reproductive anatomy and physiology is crucial for making informed management decisions and enhancing her reproductive efficiency. Several anatomical abnormalities, such as vulvar incompetence, cervical incompetence, fibrosis, uterine cysts, or ovarian tumors, can negatively impact the mare’s reproductive success.

While most of the mare’s reproductive anatomy is internal, it’s still important to have a foundational understanding of the structures involved in controlling her cycles and maintaining pregnancy, as well as the implications of potential anatomical abnormalities. Additionally, knowing the different stages of the estrus cycle will help you identify when your mare is ready for breeding

Reproductive Anatomy

Just like other mammals, the mare’s reproductive tract can be broken down into three main structures:

  • Ovaries
  • Uterus
  • Vaginal vault

Reproductive Tract of the Mare



Mares have two ovaries, which are the primary site of reproductive hormonal control. They produce sex hormones, such as estrogen and progesterone, that prepare the reproductive tract for the different stages of the estrus cycle, as well as support pregnancy.

During each estrus cycle, follicles develop on the ovaries, each containing a potential oocyte (or egg). One of these follicles, known as the dominant follicle, matures fully and releases the egg in a process called ovulation.

Following ovulation, a structure known as the corpus luteum (CL) forms at the site of the dominant follicle. [1] The corpus luteum produces progesterone, a hormone essential for maintaining pregnancy.

Each ovary is connected to the uterus by an oviduct (also called the fallopian tube in humans). If the mare is bred, sperm from the stallion travels through the reproductive tract to fertilize the egg within the oviduct. Pregnancy can be confirmed by the veterinarian at 14 days post-breeding via transrectal ultrasound. [1]

In a breeding soundness exam, your veterinarian will assess each ovary’s size, shape, and the presence of follicles or any abnormalities. Healthy, active ovaries with follicles indicate the mare is cycling normally.


If the egg is successfully fertilized, the resulting embryo will descend into the uterus around day 5-6 post-ovulation. The uterus is Y-shaped, featuring a short “body” and two branching “horns” that connect to each oviduct. The embryo migrates throughout the uterus before implanting in one of two horns at day 16-17. [1]

A healthy uterus is crucial for the implantation and development of the embryo. In a BSE, your veterinarian will evaluate the uterus using palpation and ultrasound. This assessment checks the health of the uterine lining, the presence of any cysts, fluid, or infections, and the overall condition of the uterus.


The uterine body is separated from the vagina by the cervix, a tight tubular structure that acts as a barrier between the internal and external reproductive tracts. This barrier is critical in maintaining a sterile environment within the uterus. [1]

The vaginal vault extends from the cervix to the opening of the urethra, the duct through which urine exits the body from the bladder. The vestibule is the remaining few inches of the vagina that extends from the urethral opening to the vulva. [1]

During parturition, the vagina serves as the birthing canal, allowing the foal to pass from the uterus to the outside world.

As part of a breeding soundness exam, the vagina is inspected for structural soundness and signs of infection or inflammation.

Anatomical Barriers

In addition to assessing the ovaries, uterus, and vaginal vault, several crucial structures are examined in a BSE to ensure the mare can maintain a successful pregnancy.

The mare’s reproductive tract has three anatomical barriers that prevent the spread of foreign material and bacteria from the outside world into the sterile uterus: [2][4]

  • Vulva, which should form a tight seal and prevent foreign material and manure from contaminating the vagina.
  • Vestibulovaginal sphincter, a muscular ring at the junction between the vestibule and vagina. Its main purpose is to prevent the backflow of urine into the vagina.
  • Cervix, the final barrier to the uterus within the vaginal vault. The cervix is only open for a short window in estrus or during parturition (foaling).

If any of these barriers fail, the mare becomes susceptible to developing an infection in the uterus. This can hinder her fertility success by affecting her ability to conceive or maintain a pregnancy. [1]

Estrous Cycles in the Mare

Understanding the estrous cycle is important to provide your mare with the best management and identify when she is ready for breeding. A BSE can determine if the mare is cycling normally, identify the optimal time for breeding, and detect any abnormalities that might affect her fertility.

Mares are seasonally polyestrous, which means they cycle multiple times a year during the spring and summer months. In the Northern Hemisphere, mares will typically cycle from April to October when daylight hours are longer. [1][5]

During the winter months, mares are “out of season” or in anestrus, a period of reproductive dormancy where they do not exhibit estrous cycles​. [1]

When a mare is actively cycling in the summer, her ovaries increase in size due to heightened activity, with more follicles developing. Conversely, when the mare is out of season, her ovaries become smaller and less active.

However, certain management practices can help a mare start cycling earlier in season. For example, providing mares with artificial lighting during the winter months can initiate estrus earlier than normal. [1]

Stages of the Estrous Cycle

An estrous cycle is the time period from one ovulation to the next, lasting 21 days on average. The estrous cycle in mares is divided into several distinct stages, each playing a critical role in the reproductive process: [1][6]


This is the initial phase of the estrous cycle, which lasts 2 – 5 days. During proestrus, the follicles on the ovaries begin to grow under the influence of increasing estrogen levels, preparing the mare for ovulation.


Known as the “heat” phase, estrus is the period during which estrogen levels peak and the mare is sexually receptive. This phase typically lasts for about 5 – 7 days, with ovulation occurring at the end of estrus. Signs of estrus include: [7]

  • Increased urination
  • Winking of the vulva
  • Raised tail or tail “flagging”
  • More compliant behavior when approached by the stallion


This is the post-ovulatory phase that follows estrus and lasts for approximately 14 – 15 days. During diestrus, the mare’s reproductive system prepares for a potential pregnancy. The corpus luteum (CL) produces progesterone, which maintains the uterine lining and supports early pregnancy.

If the mare does not conceive, the CL will eventually regress and the estrus cycle will begin again.

8 Steps for a Breeding Soundness Exam

With a better understanding of your mare’s reproductive system and estrous cycles, you will be better equipped to interpret the results of her breeding soundness examination. This comprehensive exam encompasses various aspects of the mare’s anatomy and physiology, ensuring she is reproductively fit.

Routine procedures performed during a BSE for mares include collecting a comprehensive history, external vulvar examination, rectal palpation and transrectal ultrasonography, vaginal speculum examination, uterine cytology and culture, and uterine biopsy. [2]

Additional procedures can be used to investigate specific or ongoing infertility issues. If these techniques are determined necessary, your veterinarian will discuss with you the best options.

In the following section we will outline eight key steps in a breeding soundness exam for mares, each of which provide vital insights into her fertility and overall reproductive status. Understanding these steps is crucial for breeders who want to optimize breeding outcomes and maintain the health of both the mare and future offspring.

1) Comprehensive History

The first step of a BSE involves gathering comprehensive information about the mare and her health history. Owners should be prepared to provide their veterinarian with details such as the mare’s age, medical and surgical history, vaccination status, performance history, and nutrition program. [4][5]

This will help your veterinarian identify potential factors that might contribute to breeding difficulties. Inadequate management can sometimes contribute to infertility.

In addition to general health and management questions, your veterinarian will ask about your mare’s reproductive history. Owners can expect questions about the following: [2][3][4][5]

  • Estrus Cycles, such as the mare’s age at first heat, date of last estrus, and typical number of days between heat cycles.
  • Breeding History, including the age at first breeding, number of breeding attempts to result in a successful pregnancy, and breeding method.
  • Pregnancy History, including date of last foaling, any difficulties foaling, mothering ability, milk production, and pregnancy loss or abortion.

2) Physical Examination

A general physical examination is conducted to check the mare’s vital signs and identify any musculoskeletal issues that could make pregnancy difficult or dangerous for the mare.

Certain conformation flaws or injuries may be exacerbated by the added weight of the growing foal during pregnancy. Examples include degenerative suspensory ligament desmitis (DSLD) and advanced arthritis in the hind end.

Breeders also aim to avoid passing undesirable traits onto offspring. If certain hereditary issues are identified on examination or through genetic testing, your veterinarian may advise against breeding. [3][5]

Your veterinarian may also examine the mare’s udder (mammary glands) to assess her potential for successful lactation. This involves checking for any defects with the teats or signs of mastitis, infection, and tumors. [10]

Your mare’s body condition can also impact her reproductive health. In general, broodmares should be maintained slightly heavier than performance horses with an ideal body condition score of 6 on the 9-point Henneke Scale.

A body condition score that is too low or too high can lead to irregular estrous cycles, reduced conception rates, and difficulty carrying a pregnancy to term. [8][11]

3) External Vulvar Examination

Assessing external vulvar conformation is an important part of the breeding soundness exam. The vulva is one of the barriers of the reproductive tract that prevents foreign material and manure from contaminating the vaginal vault.

Your veterinarian will assess the vulva’s structure, alignment, apposition, and tone. [2][5] Ideally, the vulvar lips should be vertical to the anus.

Poor conformation can lead to issues such as “wind sucking” (or air in the vagina), bacterial contamination from manure, and urine pooling. This can result in uterine infections and decrease fertility. [3][4]

Infections of the uterus can result in endometritis, a major cause of reduced fertility in the mare. [12] Reports indicate that between 10 – 25% of thoroughbred mares have endometritis and require treatment for successful breeding. [13]

If issues are identified, your veterinarian may recommend a Caslick’s procedure post-breeding to reduce the risk of bacterial infection and pregnancy loss. A Caslick’s is a routine procedure that involves suturing closed the top two-thirds of the vulva for the duration of pregnancy. The sutures are removed prior to foaling. [1][2]

4) Rectal Palpation

Your veterinarian will also perform a thorough internal examination of the reproductive tract, typically under standing sedation to ensure safety for themselves and for the mare.

This examination is performed by gently palpating the reproductive structures through the rectal wall (transrectally), which lies directly above the reproductive tract.

The veterinarian will assess the size, shape, and tone of the cervix, uterus, and ovaries. This allows the veterinarian to determine the mare’s current stage in the estrous cycle and identify abnormalities within her reproductive tract. [1][2][4]

5) Rectal Ultrasound

Following manual palpation, your veterinarian will continue with an ultrasound examination of the reproductive tract, which is also performed transrectally. This technique provides a more detailed and visual assessment of the mare’s reproductive organs.

Ultrasound examination not only verifies the stage of estrus but also enables the detection of potential abnormalities that may be difficult or impossible to identify with palpation alone.

Conditions such as fluid accumulation within the uterus, uterine cysts, or irregular structures on the ovaries can be more accurately assessed using ultrasound imaging. [2][3][4]

6) Vaginal Speculum Examination

In this examination, a speculum (a cardboard tube) is used to visually inspect the interior of the vagina. The veterinarian will look for any signs of scarring or infection. Discharge from the cervix suggests a uterine infection that will need addressed prior to breeding. [3][4]

Trauma from foaling or live cover breeding with a stallion can also result in adhesions or abnormal anatomy, which may pose a risk to normal parturition.

7) Uterine Cytology and Culture

Depending on your mare’s medical history and the findings from the examination, your veterinarian may recommend collecting samples from the uterus to check for inflammation or infection.

After thoroughly cleaning the vulva to avoid contamination, the veterinarian will insert a double-guarded swab through the vagina and cervix into the uterus. The sample collected on the swab will be cultured to determine if pathogenic bacteria are present. [2][3][4]

The swab must be covered to keep it sterile until it reaches the uterus. If the swab is accidentally contaminated by the normal population of microbes within the vagina, a false positive result may be obtained on culture. [2][3][4]

Additionally, a small brush is used to gather cell samples from the uterus for cytology. This sample is examined under a microscope for the presence of bacteria or inflammatory cells, such as neutrophils. [2][3][4]

Mares with cytology results that indicate inflammation have lower pregnancy rates than mares with normal cytology findings. [14]

8) Uterine Biopsy

If your mare is having difficulty maintaining a pregnancy, a biopsy may be recommended by your veterinarian to further evaluate the uterine tissue. In this procedure, a small sample of tissue is removed from the uterus for examination.

To perform a uterine biopsy, your veterinarian will collect a sample of the uterine lining from the base of one of the uterine horns. This tissue sample will be assessed to determine the endometrium’s ability to support a pregnancy. [2][3][4]

Collecting a tissue sample does not impact the mare’s fertility or ability to conceive, and the procedure is not painful. [1][2] The sample is evaluated for markers of inflammation and morphology of the uterine glands. [3][4]

The Kenney-Doig Scale is a grading system used to assess uterine biopsy samples from mares. It categorizes the samples into different levels based on the severity of changes observed in the uterine tissue.

  • Category I: No significant changes. Mares in this category have an 80-90% chance of carrying a foal to term.
  • Category II: Most mares fall into this category, which is divided into two subcategories. Treatment is recommended reduce inflammation and improve classification.
    • IIA: Less severe changes with a foaling rate of 50-80%.
    • IIB: More severe changes with a foaling rate of 10-50%.
  • Category III: Severe changes, often involving periglandular fibrosis, for which there is no treatment. Mares in this category have a less than 10% chance of carrying a foal to term.

Mares can improve their Kenney-Doig Scale category with treatment; however, fibrosis is a permanent condition. [4]

Management practices and other reproductive abnormalities should be considered for mares in category I or IIA that are still experiencing breeding difficulties. [2] Category IIB or III mares may still be able to produce embryos that other mares can carry to term through embryo transfer.[4]


Successfully breeding your mare is not a foregone conclusion, with foaling rates often falling below 60%. To assess a mare’s reproductive health and increase the likelihood of a successful pregnancy, ask your veterinarian to conduct a breeding soundness exam (BSE).

This comprehensive evaluation involves:

  • Gathering the mare’s medical history and reproductive background
  • Conducting a physical assessment to ensure her overall health and suitability for pregnancy
  • Thorough examination of the reproductive tract via transrectal palpation, ultrasound, and other diagnostic techniques

While not required for each mare in every breeding season, a BSE is especially important for maiden mares, senior mares, and those experiencing fertility issues. With the results of your mare’s breeding soundness exam, you can make informed decisions and take appropriate measures to support successful breeding and the birth of a healthy foal.

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  4. Johnson, A. K. Back to the Basics: Importance and interpretation of a breeding soundness exam in the mare . Auburn VetMed. 2018
  5. The Mare: Breeding Soundness Examination & Reproductive Anatomy . Cooperative Extension Service – University of Kentucky. 2020
  6. Senger, P. L . Pathways to pregnancy and parturition 3rd edition . Current Conceptions, Inc. 2012
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  13. Hemberg, E and et al. Retrospective study on vulvar conformation in relation to endometrial cytology and fertility in thoroughbred mares . J Vet Med A Physiol Pathol Clin Med. 2005
  14. Nielsen, J.M. and et al Diagnosis of Endometritis in the Mare Based on Bacteriological and Cytological Examinations of the Endometrium: Comparison of Results Obtained by Swabs and Biopsies. J Equine Vet Sci. 2010