# Equine Pregnancy Diagnosis: Methods and Clinical Application


## Key Takeaways

- Transrectal ultrasonography is the gold standard for early equine pregnancy diagnosis, enabling visualization of the embryonic vesicle from day 9-10 post-ovulation and fetal heartbeat from day 20-24, with optimal twin diagnosis between days 14-16.
- Transrectal palpation, while historically significant, is operator-dependent and best utilized from day 18-20 for uterine tone and horn asymmetry, but it cannot reliably detect twins or assess fetal viability.
- Equine chorionic gonadotropin (eCG) assays are reliable for pregnancy confirmation between days 40-120, detecting placental gonadotrophin, but can yield false negatives before day 40 or false positives in mares with persistent endometrial cups from prior pregnancies.
- Urinary or serum oestrogen assays are useful for late-stage pregnancy confirmation (after day 90) by detecting fetoplacental steroidogenesis, but are not suitable for early diagnosis or viability assessment.
- Transabdominal ultrasonography becomes feasible from day 60-90 for mares unsuitable for rectal examination, offering a means to monitor fetal well-being in later gestation.
- Accurate record-keeping, including examination date, method, findings (e.g., embryonic heart rate, fetal fluid depth), and presence of twins, is critical for effective breeding management and monitoring pregnancy loss.

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Accurate and timely pregnancy diagnosis in the mare is a core skill in equine practice, with direct consequences for breeding management, economic decisions, and the health of both mare and conceptus. This article provides a procedural reference for veterinarians performing pregnancy examinations, comparing the principal methods available: transrectal palpation, transrectal and transabdominal ultrasonography, and hormonal assays. Timing windows, diagnostic accuracy, and the clinical reasoning that guides method selection are addressed for each approach.

The reader is assumed to be a qualified practitioner familiar with equine reproductive anatomy and routine handling of the mare. The content is organized to support clinical decision-making instead of to serve as an owner education document. Where the evidence base is historical or contested, this is stated explicitly, and the reader is directed to current professional resources for updated guidance.

## At a Glance

| Parameter | Finding | Clinical Note |
|---|---|---|
| Earliest diagnosis | Transrectal ultrasound: embryonic vesicle visible from day 9 to 10 | Requires high-frequency transducer and careful technique |
| Palpation window | Uterine horn asymmetry and tone from day 18 to 20 | Subjective, dependent on operator experience |
| PMSG (eCG) detection | Reliable from day 40 to 120 | Assays detect placental gonadotrophin, false negatives possible before day 40 |
| Oestrogen assays | Urinary or serum oestrogens rise after day 90 | Useful for late confirmation, not for early diagnosis |
| Twin diagnosis | Best performed day 14 to 16 | Early identification permits manual reduction |
| Definitive non-pregnancy | No vesicle on ultrasound at day 14 to 16 with confirmed ovulation date | Repeat examination recommended |
| Fetal viability | Heartbeat visible from day 20 to 24 | Absence of heartbeat after day 25 indicates embryonic loss |

## Reproductive Physiology Relevant to Pregnancy Diagnosis

The equine conceptus enters the uterus around day 5 to 6 after ovulation and undergoes a period of extensive migration throughout the uterine lumen until fixation occurs at the base of one uterine horn, typically between day 15 and 17. This migratory phase has direct diagnostic implications. A single ultrasound examination before fixation may miss the vesicle, and repeated scanning or a thorough systematic sweep of both uterine horns is required. The vesicle grows rapidly, from approximately 3 to 4 mm at day 10 to 25 mm by day 16, which explains why early detection is feasible but technically demanding.

The endometrium responds to the presence of the conceptus through the maternal recognition of pregnancy signal, which in the mare involves the physical interaction of the migrating vesicle with the endometrium instead of a soluble embryonic signal. This mechanical signal suppresses prostaglandin F2 alpha release from the endometrium, thereby maintaining the corpus luteum. The primary luteal source of progesterone sustains the pregnancy until the endometrial cups form, beginning around day 25 to 30. The cups are specialised endometrial structures that secrete equine chorionic gonadotrophin (eCG), formerly termed pregnant mare serum gonadotrophin (PMSG). This hormone is the basis for several laboratory pregnancy tests and appears in maternal serum from approximately day 40, with peak concentrations between day 55 and 75. The historical development of these assays, including the modification of the original Cole and Hart mouse bioassay into a quantitative technique, is documented in an institutional review of laboratory methods for equine pregnancy diagnosis [Walker, institutional publication, 1977](https://pubmed.ncbi.nlm.nih.gov/405788/).

After day 70, the endometrial cups begin to regress, and eCG concentrations decline. By day 120, eCG is usually undetectable. Oestrogens, particularly oestrone sulphate produced by the fetoplacental unit, rise from around day 90 and provide a later window for hormonal confirmation. The timing of these endocrine events dictates which assay is appropriate at each gestational stage.

## Transrectal Palpation

Palpation of the reproductive tract per rectum was the standard method of pregnancy diagnosis before the widespread availability of ultrasound and remains a valuable skill. The technique relies on identifying increased uterine tone, asymmetry of the uterine horns, and eventually the bulging of the chorioallantoic sac through the uterine wall. Uterine tone increases under progesterone influence from approximately day 12, and the pregnant horn becomes more turgid and tubular than the non-pregnant horn. These changes are appreciable from day 18 to 20 in most mares, although the findings are subtle and operator-dependent.

The reliability of palpation is well established. A historical review of equine pregnancy diagnosis confirms that rectal examination is a reliable method, and it was the principal technique used in practice for decades [Walker, institutional publication, 1977](https://pubmed.ncbi.nlm.nih.gov/405788/). However, the method has limitations. It cannot detect twins reliably, it provides no information about fetal viability, and it is subject to interpretive error in mares with uterine fluid, pyometra, or a recent foal heat. The decision to use palpation alone should be reserved for situations where ultrasound is unavailable, and even then a confirmatory examination at a later date is prudent.

## Transrectal Ultrasonography

Ultrasonography is the current standard for early equine pregnancy diagnosis. Using a 5 to 7.5 MHz linear transducer, the embryonic vesicle can be identified as a discrete anechoic sphere within the uterine lumen from day 9 to 10 after ovulation. The vesicle is mobile before fixation, so the examiner must scan the entire uterine body and both horns systematically. The characteriztic appearance of the vesicle, a fluid-filled sphere with a hyperechoic capsule, becomes more obvious as it grows.

From day 20, the embryo proper is visible as an echogenic spot within the vesicle, and the heartbeat can be detected from approximately day 20 to 24. This is the earliest reliable indicator of fetal viability. The presence of a heartbeat at day 25 or later is strong evidence of a continuing pregnancy, whereas its absence at that stage indicates embryonic loss. Twin vesicles are best identified between day 14 and 16, before fixation, when both are still mobile and can be visualized separately. After fixation, twins may be hidden one behind the other, making diagnosis more difficult.

The accuracy of ultrasound at day 14 to 16, when performed by an experienced operator, approaches 100 percent for the diagnosis of pregnancy. The diagnosis of non-pregnancy is equally reliable at this stage provided the mare has a known ovulation date and the examination is complete. A mare examined before day 10 may have a vesicle too small to detect, and a repeat examination is always indicated if the result is negative but the clinical suspicion of pregnancy remains high.

## Hormonal Assays for Pregnancy Diagnosis

Hormonal testing provides a practical alternative or adjunct to imaging, particularly when ultrasound or skilled palpation is unavailable. The choice of assay depends on gestational age, sample type, and the clinical question being asked.

### Equine Chorionic Gonadotropin (ECG, formerly PMSG)

Equine chorionic gonadotropin is produced by the endometrial cups beginning around day 36 to 40 of gestation, with peak serum concentrations reached between days 55 and 70. The hormone then declines, becoming undetectable in most mares by day 120 to 150. Assays for ECG therefore have a defined diagnostic window.

Several laboratory methods have been described, including haemagglutination inhibition, latex agglutination, and enzyme-linked immunosorbent assays. Early comparisons of gonadotrophin detection methods noted variable accuracy between techniques, with some biological assays outperforming immunological ones in specific settings [Laboratory methods of equine pregnancy diagnosis](https://pubmed.ncbi.nlm.nih.gov/405788/). A comparative study of two serum gonadotrophin detection methods reported that accuracy depended heavily on the timing of sampling relative to the onset of ECG production [Equine pregnancy diagnosis. A comparison of two methods for the detection of gonadotrophin in serum](https://pubmed.ncbi.nlm.nih.gov/5816580/).

Practical limitations of ECG testing include false negatives when sampling occurs before day 40 or after day 100, and false positives in mares with persistent endometrial cups from a previous pregnancy that terminated after day 40. Mares that aborted or resorbed after endometrial cup formation may remain ECG-positive for weeks, making the test unreliable for confirming ongoing pregnancy. For this reason, ECG assays are best used as a screening test in mares with known breeding dates, with ultrasound or palpation used to confirm viability.

### Oestrogen Assays

Oestrogen concentrations rise in maternal serum and urine from approximately day 90 of gestation, reflecting fetoplacental steroidogenesis. Early work compared chemical tests for oestrogens used in equine pregnancy diagnosis, with urinary oestrogen measurement offering a non-invasive option [A comparison of the chemical tests for oestrogens used in equine pregnancy diagnosis](https://pubmed.ncbi.nlm.nih.gov/5460899/). These assays are less commonly used in modern practice, having been largely superseded by ultrasound, but they retain value in cases where the mare cannot be handled for rectal examination or when imaging equipment is unavailable.

Oestrogen assays cannot detect pregnancy before day 90 and provide no information about fetal viability beyond confirming that a functional fetoplacental unit exists. They are therefore of limited use for early diagnosis or for monitoring high-risk pregnancies.

### Progesterone Assays

Progesterone measurement is not a pregnancy test. It confirms the presence of a functional corpus luteum and the absence of luteolysis, which supports but does not prove pregnancy. A single low progesterone concentration in a mare with a known breeding date indicates that she is not maintaining a pregnancy, but a high concentration does not distinguish pregnant from non-pregnant mares with persistent luteal function or from mares in dioestrus.

Serial progesterone measurements can be useful in early pregnancy monitoring, particularly when combined with ultrasound findings. A decline in progesterone below luteal baseline before day 35 suggests impending pregnancy loss and warrants further investigation.

## Transabdominal Ultrasonography

Transabdominal ultrasound becomes feasible as the gravid uterus descends into the abdomen, typically from around day 60 to 90 depending on the mare's size and parity. A low-frequency curvilinear or sector transducer, 2 to 5 MHz, is required for adequate penetration in most adult horses. The examination is performed from the ventral abdomen, with the mare standing and the transducer placed cranial to the udder and lateral to the midline.

The technique is most useful for confirming pregnancy in mares that cannot undergo rectal palpation or rectal ultrasound, including miniature breeds, donkeys, or mares with rectal tears or strictures. It also provides a means of monitoring fetal heart rate and fetal fluid depth in late gestation. Transabdominal imaging does not reliably detect pregnancy before day 60 and provides less detailed assessment of endometrial health and embryonic morphology than transrectal ultrasound.

## Method Selection and Diagnostic Sequence

The choice of diagnostic method depends on gestational age, available equipment, mare temperament, and the clinical indication for the examination. The following table compares the principal methods.

| Method | Earliest Reliable Detection | Optimal Window | Sensitivity | Practical Considerations |
|---|---|---|---|---|
| Transrectal palpation | Day 30 to 35 | Day 35 to 65 | High in experienced hands | Requires skilled examiner, cannot assess fetal viability, risk of rectal injury |
| Transrectal ultrasound | Day 10 to 14 | Day 14 to 30 | Very high | Detects embryonic heartbeat from day 21, assesses twins and endometrial health |
| ECG assay | Day 40 | Day 55 to 70 | High within window | False positives after pregnancy loss, no viability information |
| Oestrogen assay | Day 90 | Day 120 to term | Moderate | Confirms fetoplacental unit, no early diagnosis |
| Transabdominal ultrasound | Day 60 to 90 | Day 90 to term | Moderate to high | Useful when rectal examination is contraindicated, limited early assessment |

The Society for Theriogenology provides professional resources on reproductive management and breeding soundness evaluation that can guide protocol development in practice [Society for Theriogenology resources](https://www.therio.org/). General clinical guidance on reproductive examination technique is also available through the [MSD Veterinary Manual](https://www.msdvetmanual.com/).

For a standard early pregnancy examination in a broodmare, transrectal ultrasound at day 14 to 16 confirms pregnancy, identifies twins, and allows assessment of the corpus luteum. A follow-up examination at day 25 to 30 confirms embryonic viability by detection of a heartbeat and monitors for early embryonic loss. If ultrasound is unavailable, transrectal palpation at day 35 to 45 provides reliable diagnosis but cannot detect twins or confirm viability.

In mares with unknown breeding dates or suspected pregnancy loss, ECG testing after day 40 can confirm that a pregnancy was established, but a positive result does not confirm an ongoing pregnancy. Transabdominal ultrasound after day 90 provides the most reliable confirmation of continued gestation in these cases.

## Documentation and Monitoring Parameters

Accurate record keeping is essential for managing breeding programs and for medicolegal purposes. Each pregnancy examination should record the mare's identity, breeding dates, examination date, method used, and findings in a standardized format. For ultrasound examinations, include embryonic or fetal heart rate, fetal fluid depth, and any abnormalities of the endometrium or conceptus.

Serial examinations provide the most useful monitoring data. A normal embryonic heartbeat at day 21 to 25 ranges from 120 to 180 beats per minute, with rates increasing as gestation advances. Fetal heart rate below 90 beats per minute after day 30 warrants concern and repeat evaluation. Fetal fluid depth increases progressively, and subjective assessment of fluid echogenicity can identify placentitis in late gestation.

Documentation should also note the presence or absence of twins, the location of the conceptus within the uterine horn, and any endometrial pathology such as cysts or fluid accumulation. This information guides subsequent management decisions, including the need for twin reduction or treatment of endometritis.

## Limitations and Sources of Error

Every diagnostic method has defined failure modes. Transrectal palpation can miss pregnancy before day 30, and uterine tone in early pregnancy can be mimicked by dioestrus or by uterine fluid accumulation. Ultrasound examination before day 10 may fail to detect a vesicle, and an embryonic vesicle can be missed if the uterus is not scanned systematically from the bifurcation through both horns. ECG assays produce false negatives when sampling falls outside the production window and false positives after pregnancy loss [Laboratory methods of equine pregnancy diagnosis](https://pubmed.ncbi.nlm.nih.gov/405788/).

Operator experience is the single largest variable in both palpation and ultrasound accuracy. Practitioners should calibrate their own sensitivity and specificity against a reference standard, such as ultrasound confirmation at day 30 or foaling outcome, and adjust their examination protocols accordingly. Where doubt exists, repeat the examination at an interval appropriate to the method instead of relying on a single inconclusive result.

## Complications and Failure Modes

Pregnancy loss is the most consequential complication in equine pregnancy diagnosis. Early embryonic death occurs before day 35 in 5 to 10 percent of confirmed pregnancies, and the rate rises with maternal age, uterine pathology, and progesterone insufficiency. A mare diagnosed pregnant at day 14 may be non-pregnant at day 25 without any outward sign. The only reliable detection method is re-examination at the appropriate interval. A mare with a regressing corpus luteum and declining progesterone may maintain a non-viable conceptus for days, so a single low progesterone value does not confirm loss. Conversely, a normal progesterone concentration does not guarantee viability.

Twin pregnancy is a second major failure mode. Twins are diagnosed reliably by transrectal ultrasonography between days 14 and 16, when two vesicles are visible as separate anechoic spheres. After day 16, vesicles may fuse or become superimposed, making the diagnosis more difficult. Palpation cannot reliably identify twins before day 30 and may miss them entirely. When twins are suspected but not confirmed, re-scan at 48-hour intervals until day 18. If twin vesicles are found, the clinician must decide between manual reduction, transvaginal ultrasound-guided aspiration, or continued monitoring, depending on the stage and the clinician's skill. Untreated twins carry a high risk of abortion in late gestation.

Mummification and maceration are late complications that may be detected incidentally. A mummified fetus presents as a firm mass on palpation with no fetal fluids detectable by ultrasound. The mare may remain behaviorally pregnant with a closed cervix. Maceration follows fetal death with cervical relaxation and ascending infection, producing a purulent vaginal discharge and systemic signs. Both conditions require prompt intervention and carry a guarded prognosis for future fertility.

| Observation | Likely cause | Discriminating check |
|---|---|---|
| Vesicle present at day 14, absent at day 25 | Early embryonic death | Re-scan at day 25 to day 30, assess uterine tone and progesterone |
| Two vesicles seen at day 14, one at day 18 | Twin reduction or fusion | Serial scanning at 48-hour intervals, note vesicle position and shape |
| Positive ECG test, no vesicle on ultrasound | Recent pregnancy loss or ectopic pregnancy | Repeat ultrasound in 5 to 7 days, measure progesterone |
| Uterine tone increased, no vesicle visible | Endometritis or early pregnancy before day 10 | Re-scan after day 12, consider cytology and culture |
| Fetal fluids present, no fetal heartbeat after day 25 | Embryonic death with retained fluids | Doppler or M-mode ultrasound, re-check in 48 hours |

## Common Errors and Corrective Action

The most frequent error in early pregnancy diagnosis is scanning too early. Before day 10, the conceptus may be migrating within the uterus and can be missed even with high-quality equipment. Scanning before day 12 and declaring the mare non-pregnant is a preventable mistake. The corrective action is to establish a minimum scanning age of day 14 for a definitive negative diagnosis and to document the date of ovulation, not the date of breeding, as the reference point.

A second error is misidentifying uterine fluid as a conceptus. Uterine edema, which peaks during oestrus, produces a characteriztic endometrial folding pattern on ultrasound. A fluid pocket within edematous endometrium can mimic a vesicle. The discriminating feature is shape and location: a true vesicle is spherical with a distinct anechoic lumen, whereas edema fluid is irregular and follows the endometrial folds. Rotating the transducer and scanning in both transverse and longitudinal planes resolves most ambiguity.

Palpation errors arise from confusing the uterine body with the cervix or from mistaking a full bladder for a gravid horn. A mare with a distended bladder can push the uterus cranially, making the uterine horns difficult to reach. Emptying the bladder by catheterization or waiting 30 minutes before re-examination is the corrective action. Less experienced clinicians also tend to overestimate uterine tone in the non-pregnant mare with endometritis, leading to a false positive palpation diagnosis. Ultrasound is the definitive arbiter in all such cases.

## Limitations of the Evidence and Divergent Expert Opinion

The published evidence on equine pregnancy diagnosis spans several decades, and much of the foundational work predates modern ultrasonography. The laboratory methods described by Walker in 1977, including the mouse uterine weight bioassay for equine chorionic gonadotropin, are of historical interest but have been superseded by rapid immunoassays and ultrasound [Laboratory methods of equine pregnancy diagnosis](https://pubmed.ncbi.nlm.nih.gov/405788/). Similarly, the comparison of chemical tests for oestrogens by Cox and Galina reflects an era when urinary oestrogen assays were a practical option [A comparison of the chemical tests for oestrogens used in equine pregnancy diagnosis](https://pubmed.ncbi.nlm.nih.gov/5460899/). These sources establish the accuracy of biological and chemical methods but do not provide contemporary performance data for ultrasound or modern hormone assays.

Expert opinion still differs on the optimal timing for the first pregnancy examination. Some clinicians advocate scanning at day 12 to allow earlier twin reduction, while others prefer day 14 to day 16 to reduce the risk of missing a vesicle. Both approaches are defensible, and the choice depends on the clinician's ultrasound skill and the mare's value. There is also disagreement about the clinical utility of a single progesterone measurement in early pregnancy. Progesterone assays are reliable for confirming luteal function but cannot distinguish pregnancy from a persistent corpus luteum, and the [MSD Veterinary Manual](https://www.msdvetmanual.com/) notes that progesterone measurement is most useful when interpreted alongside ultrasound findings.

## Referral, Consultation, and Reporting

Referral to a specialist theriogenologist is warranted when twin reduction is attempted beyond day 16, when the clinician cannot confidently identify or exclude twins, or when repeated early embryonic death suggests uterine pathology requiring hysteroscopy or biopsy. Laboratory involvement is indicated when hormonal assays are needed to confirm pregnancy in mares where ultrasound is inconclusive, such as in late gestation when the fetus is beyond the reach of transrectal imaging. The [Society for Theriogenology](https://www.therio.org/) provides resources for locating board-certified specialists and for continuing education in advanced reproductive procedures.

Regulatory reporting is rarely required for routine pregnancy diagnosis. However, the presence of a mummified fetus, unexplained abortion, or vesicular lesions on the placenta may trigger reportable disease obligations under national or international standards. The [WOAH terrestrial animal health standards](https://www.woah.org/en/what-we-do/standards/codes-and-manuals/terrestrial-code-online-access/) list equine diseases with surveillance and notification requirements, and practitioners should consult their regional veterinary authority when such findings arise. The [AVMA practice resources](https://www.avma.org/resources-tools) offer guidance on professional obligations and record-keeping in these circumstances.

## Frequently Asked Questions

### How should I proceed when transrectal ultrasonography is unavailable?

Transrectal palpation remains a reliable diagnostic method when ultrasound is not an option, particularly between days 35 and 60 of gestation when the chorioallantoic sac is readily palpable. Palpation findings must be interpreted with attention to uterine symmetry, tone, and the presence of an embryonic bulge. For earlier detection, an ECG assay on serum collected between days 45 and 90 provides a practical alternative, and laboratory-based methods have been described with defined accuracy criteria including sensitivity and a minimal inconclusive rate [Walker, laboratory methods of equine pregnancy diagnosis](https://pubmed.ncbi.nlm.nih.gov/405788/). When relying on palpation alone, schedule a follow-up examination at day 60 to confirm ongoing pregnancy, as early embryonic loss cannot be excluded by a single positive finding.

### What is the most cost-effective testing strategy for a large broodmare band?

For herd-level screening, ECG testing on serum collected between days 45 and 60 offers the most economical approach when ultrasound examination of every mare is impractical. This window coincides with peak placental gonadotrophin concentration, and laboratory-based assays have been developed specifically to meet requirements for accuracy, sensitivity, and minimal subjective interpretation [Walker, laboratory methods of equine pregnancy diagnosis](https://pubmed.ncbi.nlm.nih.gov/405788/). Mares that test positive can be scheduled for a confirmatory ultrasound at day 60 to assess fetal viability and detect twins. Mares that test negative should be re-examined or re-tested, since sampling outside the optimal window or assay failure can produce false negatives. Compare the cost of batch testing against the labor and equipment costs of individual ultrasound examinations before selecting a protocol.

### When should I recommend transabdominal ultrasonography over transrectal imaging?

Transabdominal ultrasonography becomes the preferred approach after approximately day 60, when the gravid uterus descends beyond reliable transrectal reach. It is also indicated when transrectal examination is contraindicated, such as in mares with severe rectal tone, recent rectal tears, or fractious behavior that makes restraint hazardous. A low-frequency convex transducer placed in the ventral flank region provides adequate penetration for fetal imaging. Transabdominal imaging permits assessment of fetal heart rate, fetal movement, and placental character in later gestation, information that transrectal ultrasound cannot provide. For early pregnancy diagnosis, transrectal imaging remains superior for detecting the embryonic vesicle and confirming viability before day 30. The two modalities are complementary instead of interchangeable.

### How do I interpret a positive ECG result in a mare that subsequently fails to maintain pregnancy?

A positive ECG result confirms that placental gonadotrophin was present in serum, which indicates that functional endometrial cups had formed. It does not confirm that the pregnancy is currently viable, because endometrial cups persist and continue secreting ECG even after embryonic death has occurred. This limitation is well recognized in the literature on laboratory pregnancy diagnosis methods [Walker, laboratory methods of equine pregnancy diagnosis](https://pubmed.ncbi.nlm.nih.gov/405788/). When a mare tests positive but later shows no pregnancy on ultrasound, explain to the owner that embryonic loss occurred after endometrial cup formation, typically after day 36. The mare may continue to show positive ECG results until the cups regress around day 120, so re-testing is not useful for confirming ongoing pregnancy during this period.

### What records should I maintain for pregnancy diagnosis examinations?

Record the mare identification, breeding dates, each examination date, the method used, and the findings in a standardized format. For ultrasound examinations, document the presence or absence of an embryonic vesicle, its diameter or crown-rump length, fetal heart rate when measured, and the number of vesicles or fetuses identified. For palpation, record uterine tone, symmetry, and the location and size of any palpable bulge. For hormonal assays, record the assay type, the laboratory, the sample date, and the result. These records support monitoring of embryonic loss rates within a breeding program and provide a basis for comparing diagnostic methods across seasons. Professional resources on reproductive health management emphasize the value of systematic record keeping for breeding soundness evaluation and herd-level reproductive monitoring [Society for Theriogenology resources](https://www.therio.org/).

### How should I explain a false negative result to a client?

A false negative result means the test failed to detect a pregnancy that is present. Explain that no diagnostic method is perfect, and that timing errors account for most false negatives. For ECG assays, sampling before day 40 or after day 90 can produce a negative result despite an ongoing pregnancy, because gonadotrophin concentrations fall outside the detectable range. For ultrasound, an examination performed before day 12 may miss a vesicle that is simply too small to resolve. Emphasize that the recommended next step is a repeat examination at the appropriate gestational age, not a change in management based on a single negative result. This approach reflects the published emphasis on accuracy and minimal inconclusive rates in laboratory pregnancy diagnosis methods [Walker, laboratory methods of equine pregnancy diagnosis](https://pubmed.ncbi.nlm.nih.gov/405788/).

## Related Clinical & Scientific Guides

* [Diagnostic Approach to Canine Infertility in the Bitch](/knowledge/veterinary-medicine/theriogenology/diagnostic-approach-to-canine-infertility-in-the-bitch)
* [Canine Neonatal Resuscitation: Protocol and Monitoring](/knowledge/veterinary-medicine/theriogenology/canine-neonatal-resuscitation-protocol-monitoring)
* [Equine Breeding Soundness Examination of the Stallion](/knowledge/veterinary-medicine/theriogenology/equine-breeding-soundness-examination-of-the-stallion)


## References and Further Reading

- [Laboratory methods of equine pregnancy diagnosis.](https://pubmed.ncbi.nlm.nih.gov/405788/). 1977.
- [A comparison of the chemical tests for oestrogens used in equine pregnancy diagnosis.](https://pubmed.ncbi.nlm.nih.gov/5460899/). 1970.
- [Equine pregnancy diagnosis.](https://pubmed.ncbi.nlm.nih.gov/15431504/). 1950.
- [Equine pregnancy diagnosis. A comparison of two methods for the detection of gonadotrophin in serum.](https://pubmed.ncbi.nlm.nih.gov/5816580/). 1969.
- [Laboratory methods of equine pregnancy diagnosis.](https://pubmed.ncbi.nlm.nih.gov/878277/). 1977.
- [Society for Theriogenology Resources](https://www.therio.org/). Society for Theriogenology.
- [MSD Veterinary Manual, Professional Edition](https://www.msdvetmanual.com/). MSD Veterinary Manual.
- [American Veterinary Medical Association Practice Resources](https://www.avma.org/resources-tools). American Veterinary Medical Association.
- [WOAH Terrestrial Animal Health Code](https://www.woah.org/en/what-we-do/standards/codes-and-manuals/terrestrial-code-online-access/). WOAH.

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> This article is educational professional reference material for veterinary audiences. It is not a substitute for veterinary diagnosis, individual clinical judgment, current product labeling, or applicable regulatory requirements.