# Ovine Pregnancy Diagnosis: Transabdominal Ultrasound and Management


## Key Takeaways

- Transabdominal ultrasonography is the primary method for ovine pregnancy diagnosis, with optimal scanning occurring between days 25 and 90 of gestation, and highest accuracy from day 30 onward.
- Key positive ultrasound findings include an anechoic fluid-filled uterine lumen (visible from day 25), a detectable fetal heartbeat (from day 30), and placentomes (visible from day 35-40), which can aid in litter size estimation.
- Fetal number estimation is most accurate between days 40 and 60, with accuracy declining significantly after day 70 due to fetal overlap; a negative diagnosis requires systematic scanning of both uterine horns.
- Common diagnostic challenges include differentiating the pregnant uterus from the urinary bladder or rumen, and false negatives can result from scanning too early or inadequate transducer contact.
- Pregnancy-associated glycoprotein (PAG) assays serve as a confirmatory test for equivocal ultrasound findings, detecting pregnancy from approximately day 20 in plasma.
- Potential complications include early embryonic loss, which can lead to a positive scan followed by failure to lamb, and fetal abnormalities suggestive of conditions like Border disease, warranting further laboratory investigation.

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Transabdominal ultrasonography is the dominant method for pregnancy diagnosis in commercial sheep practice, offering rapid, accurate, and cost-effective results across large flocks. This article provides a procedural reference for veterinarians performing transabdominal scanning in ewes, covering the physical principles of the technique, optimal timing relative to gestation, image interpretation, and the clinical decisions that follow a positive or negative diagnosis. The focus is strictly on the ultrasound examination itself and its immediate diagnostic consequences, not on broader flock nutrition or long-term management programs.

The intended reader is a practicing veterinarian who may be new to ovine scanning or who wishes to refine their technique and interpretive framework. The article answers the practical questions of when to scan, what equipment to use, how to position the ewe, what findings confirm pregnancy at each gestational stage, and how to distinguish pregnancy from the common artifacts and pathological conditions that mimic it. Where the evidence base is limited or contested, this is stated explicitly.

Accurate pregnancy diagnosis is the foundation of reproductive management in sheep flocks. A false negative leads to ewes being carried through a non-productive year, while a false positive results in the culling or treatment of a ewe that is not pregnant. The predictive values of any diagnostic test depend on the prevalence of pregnancy in the population being tested, and this must be considered when interpreting results in early-season or late-breeding groups.

## At a Glance

| Parameter | Value or Decision | Notes |
|---|---|---|
| Optimal scanning window | Day 25 to 90 of gestation | Accuracy is highest from day 30 onward |
| Transducer frequency | 3.5 to 5 MHz for transabdominal use | Lower frequencies penetrate deeper but reduce resolution |
| Ewe positioning | Standing or dorsal recumbency | Standing is faster, recumbency improves access in obese ewes |
| Key positive finding | Anechoic fluid in the uterine lumen | Visible from approximately day 25 |
| Fetal heartbeat | Detectable from approximately day 30 | Confirms viability |
| Placentomes | Visible from approximately day 35 to 40 | Counts correlate with litter size but are not exact |
| Fetal number estimation | Best performed between day 40 and 60 | Accuracy declines after day 70 as fetuses overlap |
| Negative diagnosis | Requires scanning both uterine horns | A single horn may be missed in early gestation |
| Confirmatory test | Pregnancy-associated glycoprotein (PAG) assay | Useful when ultrasound is inconclusive |

## Physics and Equipment for Transabdominal Scanning

Real-time B-mode ultrasound relies on the reflection of sound waves at tissue interfaces. In the pregnant ewe, the fluid-filled uterine lumen provides a strongly anechoic target that contrasts sharply with the surrounding soft tissue. This acoustic window is the basis of transabdominal diagnosis, and it becomes more reliable as gestation advances and the volume of fetal fluid increases.

A 3.5 to 5 MHz convex transducer is standard for transabdominal ovine scanning. The lower frequency penetrates the abdominal wall and rumen, while the convex footprint allows a wide field of view across the gravid uterus. Linear transducers may be used in thin ewes but offer a narrower window and are less forgiving of poor contact. The transducer face must be coupled with a suitable gel or, in some field settings, a water-based lubricant. Wool over the scanning site should be parted or clipped, wetting the fleece with alcohol or water improves acoustic coupling without clipping in many cases.

The examination is performed with the ewe standing in a restraint chute or held by an assistant. The transducer is placed on the ventral abdomen, cranial to the udder and lateral to the midline, and angled dorsally and cranially. The rumen occupies the left side of the abdomen and can obscure the uterus in some positions, so scanning from the right side is often more productive. In obese ewes or those with a heavy fleece, dorsal recumbency with the hindlimbs extended may be necessary to bring the uterus within reach of the transducer.

## Gestational Timeline and Diagnostic Windows

The accuracy of transabdominal ultrasound is time-dependent. Before day 25, the conceptus is small and the uterine fluid volume is insufficient for reliable detection. From day 25 onward, the fluid-filled uterine lumen becomes visible as a discrete anechoic structure, and the predictive value of a positive test rises accordingly. Early work using real-time ultrasound, initially via the rectal route, demonstrated that pregnancy could be detected from day 25 with a predictive value of a positive test of 97% and an overall accuracy of 91% [Buckrell and colleagues, real-time ultrasound rectally for early pregnancy diagnosis in sheep](https://pubmed.ncbi.nlm.nih.gov/16726157/). Transabdominal scanning achieves similar accuracy from approximately day 30.

The optimal commercial scanning window is day 30 to 60. Within this period, fetal number estimation is most reliable. After day 70, fetal growth causes the conceptuses to overlap, and counting individual fetuses becomes progressively less accurate. Scanning before day 30 is possible but requires higher-frequency transducers and greater operator experience, and the rate of false negatives is higher.

## Interpretation of Ultrasound Findings

The first positive finding is an anechoic fluid pocket within the uterine lumen, typically visible from day 25 to 30. This must be distinguished from the fluid-filled bladder, which lies more caudally and is not associated with the uterine wall. The pregnant uterus appears as a tubular or oval structure with a distinct wall, and it is usually located cranially to the bladder.

From day 30, the embryo becomes visible as a small echogenic mass within the fluid. The fetal heartbeat is the most reliable indicator of viability and is usually detectable by day 30 to 35. By day 35 to 40, placentomes appear as small echogenic nodules along the uterine wall. Their number and distribution can be used to estimate litter size, but the correlation is imperfect, and the count should be treated as a guide instead of an exact figure.

Fetal number estimation is best performed between day 40 and 60. The operator counts the number of fetal heads or the number of distinct fetal masses within the uterine lumen. Multiple pregnancies are identified by the presence of more than one fetus in separate fluid pockets or by the observation of more than one head in a single scan plane. The accuracy of this count declines after day 70, and it is not reliable after day 90.

## Differential Diagnoses and Artifacts

The most common cause of a false positive is the presence of a fluid-filled viscus other than the uterus. The bladder is the usual culprit, and it can be identified by its caudal position, thin wall, and absence of fetal structures. A distended rumen or cecum may also produce anechoic regions, but these are typically more dorsal and lack the organized structure of the gravid uterus.

A false negative most often results from scanning too early, from poor transducer contact, or from failure to scan both uterine horns. In early gestation, the conceptus may lie in one horn only, and a single scan plane through the contralateral horn will miss it. The operator must systematically sweep the entire ventral abdomen to ensure both horns are examined.

Uterine pathology can complicate interpretation. Hydrometra, pyometra, and mummified fetuses may produce fluid-filled structures that mimic pregnancy. The absence of a fetal heartbeat and the lack of placentomes help distinguish these conditions from a viable pregnancy. In cases where ultrasound findings are equivocal, a pregnancy-associated glycoprotein assay can provide a confirmatory result, as these proteins are detectable in maternal plasma from approximately day 20 of gestation [El Amiri and colleagues, pregnancy-associated glycoprotein concentration in plasma and milk for early pregnancy diagnosis in Lacaune dairy sheep](https://pubmed.ncbi.nlm.nih.gov/25613086/).

## Step-by-Step Transabdominal Scanning Protocol

Position the ewe in a standing or sitting restraint. A sitting position, with the ewe held upright against the handler's legs, provides the most stable access to the ventral abdomen. Clip a small window of wool from the midline, cranial to the udder and caudal to the xiphoid. Apply coupling gel generously. Use a 3.5 to 5 MHz convex transducer for mid to late gestation. For ewes scanned before day 40, a 5 to 7.5 MHz linear or microconvex probe improves resolution of small fluid-filled uterine horns.

Begin the scan in the caudal ventral abdomen, just cranial to the pelvic brim. Angle the beam dorsally and cranially. Identify the urinary bladder as an anechoic spherical structure. The pregnant uterus appears dorsal or lateral to the bladder. Sweep the transducer systematically from midline to both flanks, covering the full width of the abdomen. In ewes carrying twins or triplets, the gravid uterus extends further cranially, so continue the sweep to the level of the umbilicus.

Each examination should take less than one minute once the operator is proficient. Real-time transrectal ultrasound has been described as a rapid technique requiring under one minute per ewe, with a positive predictive value of 97% and overall accuracy of 91% when performed from day 25 after breeding. Transabdominal scanning is slower but remains practical for throughput of 100 to 200 ewes per hour with experienced personnel and a well-designed handling race.

Record the following for each ewe: ewe identification, gestational age or estimated days, number of fetuses, fetal heart rate if assessed, and any abnormal findings. Use a standardized form or electronic record. Include the transducer frequency, image quality, and any limitations such as excessive abdominal fat or poor contact.

## Fetal Age Estimation Reference Table

Fetal age estimation guides management decisions including vaccination timing, shearing schedules, and predicted lambing dates. The following table provides reference values for transabdominal ultrasound findings.

| Gestational Age (days) | Key Ultrasound Findings | Measurement or Feature |
|------------------------|------------------------|------------------------|
| 20 to 25 | Fluid-filled uterine lumen, embryonic vesicle may be visible | Vesicle diameter 5 to 15 mm |
| 30 to 35 | Embryo visible, heartbeat detectable | Crown-rump length 15 to 25 mm |
| 40 to 50 | Fetal limbs and head distinguishable | Crown-rump length 40 to 70 mm |
| 50 to 60 | Ribs visible, fetal movement frequent | Crown-rump length 70 to 110 mm |
| 60 to 75 | Skeletal ossification evident, placentomes visible | Femur length 15 to 30 mm |
| 75 to 90 | Fetal organs well defined, wool follicle development | Biparietal diameter 25 to 35 mm |
| 90 to 120 | Fetal size large, position stable | Crown-rump length 250 to 400 mm |
| 120 to term | Fetal orientation and presentation assessable | Amniotic fluid volume decreases |

Crown-rump length is most accurate before day 60. After day 75, biparietal diameter and femur length provide more reliable estimates. Placentome size increases with gestational age but varies considerably between individuals and with litter size. Fetal age estimation beyond day 100 is imprecise, and the clinician should state the confidence interval when reporting dates.

## Decision Points and What Changes the Decision

The primary decision after scanning is whether the ewe is pregnant, open, or has a nonviable pregnancy. A positive diagnosis requires identification of a fetus with a heartbeat or, before day 30, a clearly defined embryonic vesicle with a visible embryo. A negative diagnosis before day 30 is unreliable. Transrectal scanning from day 25 gives a negative predictive value of 80%, meaning one in five ewes diagnosed open may actually be pregnant. For transabdominal scanning, defer the final open diagnosis until day 45 to 50.

Fetal number determination is most accurate between days 45 and 75. Before day 40, embryos may be missed due to small size. After day 90, fetal crowding and overlapping structures reduce accuracy. If the ewe is carrying more than two fetuses, the risk of pregnancy toxemia and dystocia increases, and the clinician should recommend closer monitoring. The decision to intervene, for example by inducing lambing or providing supplemental nutrition, depends on fetal count and gestational age.

A diagnosis of fetal death changes the management plan. Ultrasound findings include absence of a heartbeat, loss of fetal tone, and eventually collapse of the thoracic cavity and loss of recognizable fetal structures. If fetal death occurs early, the ewe may resorb the pregnancy or abort. Late fetal death requires monitoring for metritis and retained fetal membranes. The clinician should re-scan in 7 to 10 days if fetal viability is uncertain.

## Equipment and Consumable Choices

The choice of transducer frequency balances penetration against resolution. A 3.5 MHz probe penetrates to 15 to 20 cm, sufficient for obese ewes or late gestation. A 5 MHz probe provides better resolution to 8 to 12 cm depth and suits most ewes. A 7.5 MHz probe is reserved for early pregnancy or thin ewes. Linear probes give a wider near-field image, while convex probes offer a broader sector view. Sector or phased-array probes are useful for imaging between the ribs but are rarely needed in sheep.

Coupling gel should be warmed to body temperature to reduce ewe movement. Use a gel with high viscosity to prevent runoff on wool. Clip the scanning window closely to improve contact. In cold weather, protect the transducer from thermal shock and keep spare batteries warm. Portable ultrasound machines with a 12 to 15 cm screen are adequate for field use. A machine with a recording function allows review of images and documentation for the client.

The scanning environment affects throughput. A race with a solid side and a head gate allows the operator to work safely from behind. A non-slip floor reduces the risk of falls. For large flocks, set up a dedicated scanning station with power supply, gel warmer, and a table for records. For small flocks, a lambing pen or stable works adequately.

## Documentation and Reporting

Document each ewe's pregnancy status, fetal number, and estimated gestational age. Use a three-category classification: pregnant, open, or uncertain. For uncertain cases, state the reason, such as early gestational age or poor image quality, and recommend a re-scan date. Record fetal number as single, twin, triplet, or greater. If fetal number cannot be determined with confidence, record the minimum number visible and note the limitation.

Provide the client with a written summary that includes the scanning date, the estimated lambing window, and any ewes requiring follow-up. For ewes diagnosed open, recommend a re-check if the scan was performed before day 45. For ewes with suspected fetal death, schedule a re-scan and discuss monitoring for complications. The Society for Theriogenology provides professional resources on reproductive health management that can support practice protocols and client communication.

Image storage is valuable for medicolegal purposes and for tracking accuracy over time. Store representative images for each ewe, particularly for uncertain diagnoses. Compare scanning results with lambing records to audit the accuracy of the technique and identify systematic errors. This audit is especially important for practitioners new to ovine scanning or when using unfamiliar equipment.

## Recognized Complications and Failure Modes

Transabdominal scanning in ewes fails most often from avoidable technical causes, but several biological complications deserve specific attention. Early embryonic loss is the most common cause of a positive scan that later fails to produce lambs. A ewe scanned between days 25 and 35 may show a viable embryo that subsequently resorbs, and the reported predictive value of a positive test at this stage is 97%, meaning roughly 3 in 100 positive diagnoses will not carry to term [Real-time ultrasound rectally for early pregnancy diagnosis in sheep](https://pubmed.ncbi.nlm.nih.gov/16726157/). Re-scanning suspect ewes after day 45 reduces this error.

Border disease should be considered when scanning reveals fetal abnormalities, small-for-date lambs, or when a flock has concurrent cattle with bovine viral diarrhea virus. Transmission from persistently infected cattle to pregnant sheep during mid-gestation produces the classic hairy-shaker lamb phenotype and high perinatal mortality [Border disease in sheep caused by transmission of virus from cattle persistently infected with bovine virus diarrhea virus](https://pubmed.ncbi.nlm.nih.gov/1851350/). Ultrasound cannot diagnose border disease directly, but it can identify fetal size discrepancies and abnormal placentomes that warrant laboratory investigation.

Large offspring syndrome, an epigenetic overgrowth condition, has been reported spontaneously in ruminants and is associated with macrosomia, abdominal wall defects, and dystocia [Large offspring syndrome in ruminants: current status and prediction during pregnancy](https://pubmed.ncbi.nlm.nih.gov/37567678/). Affected fetuses may appear oversized for gestational age on late scans, but current imaging tools lack the accuracy to predict this condition reliably. When fetal oversize is suspected, plan for assisted delivery and have a caesarean contingency.

## Common Operator Errors and Corrections

The most frequent error in early scanning is misidentifying the fluid-filled bladder or rumen as a pregnant uterus. The bladder is ventral and anechoic but lacks the echogenic fetal structures and placentomes. The rumen contains particulate matter that produces scattered echoes, unlike the uniform anechoic fluid of the allantoic sac. Sweep the transducer systematically from the caudal udder region cranially and identify the uterus by its tubular shape and the presence of fetal membranes.

A second common error is scanning too early and reporting a negative result. Transabdominal detection of pregnancy is unreliable before day 25, and even experienced operators will miss a proportion of pregnancies at this stage. The predictive value of a negative test at day 25 is 80%, so a negative result at this time should be followed by a repeat scan after day 35 [Real-time ultrasound rectally for early pregnancy diagnosis in sheep](https://pubmed.ncbi.nlm.nih.gov/16726157/). Do not certify a ewe as open on a single early scan.

Inadequate transducer contact is a third frequent problem. The wool over the ventral abdomen must be parted or clipped, and coupling gel applied generously. Pressure must be firm enough to displace rumen gas but gentle enough to avoid fetal trauma. Operators who rush the examination or scan through heavy fleece will produce nondiagnostic images and false negatives.

## Troubleshooting Table

| Observation | Likely Cause | Discriminating Check |
|---|---|---|
| No fluid-filled structure seen | Scan too early, transducer too cranial, or ewe open | Repeat scan after day 35, reposition transducer caudal to udder |
| Large anechoic structure, no fetus | Bladder or rumen misinterpreted | Identify fetal heartbeat or placentomes, bladder is caudal and midline |
| Fetal size smaller than expected | Incorrect breeding dates, fetal growth restriction, or border disease | Review breeding records, assess placentomes, consider PAG assay |
| Fetal oversize near term | Large offspring syndrome, incorrect dates, or multiple pregnancy | Count fetuses, estimate fetal age from crown-rump length, plan dystocia management |
| Positive scan, no lamb at term | Early embryonic loss | Document scan date, rescan after day 45 before certifying pregnancy |

## Evidence Limitations and Expert Divergence

The evidence base for ovine transabdominal ultrasound is thinner than for cattle. Much of the early validation work used rectal transducers, and direct extrapolation to transabdominal technique requires caution [The use of real-time ultrasound rectally for early pregnancy diagnosis in sheep](https://pubmed.ncbi.nlm.nih.gov/16726157/). Pregnancy-associated glycoprotein assays offer a complementary blood or milk test from day 20 in plasma and day 28 in milk, but these assays are not universally available and their performance varies between test systems [Pregnancy-associated glycoprotein concentration in plasma and milk samples for early pregnancy diagnosis in Lacaune dairy sheep](https://pubmed.ncbi.nlm.nih.gov/25613086/). Expert opinion differs on the optimal scanning window: some practitioners favour a single scan at day 45 to 60 for both pregnancy and fetal counting, while others prefer an early scan at day 30 for culling decisions followed by a later count. Both approaches are defensible, and the choice depends on flock economics and the operator's skill level.

## Referral and Reporting Thresholds

Referral or specialist consultation is warranted when fetal abnormalities are detected, when fetal count cannot be determined reliably in a ewe carrying multiple lambs, or when dystocia is anticipated from fetal oversize. Laboratory involvement is indicated when border disease is suspected, particularly when flocks have contact with cattle, because virus isolation and serology confirm the diagnosis [Border disease in sheep caused by transmission of virus from cattle persistently infected with bovine virus diarrhea virus](https://pubmed.ncbi.nlm.nih.gov/1851350/). Pregnancy-associated glycoprotein testing may be used to confirm equivocal ultrasound findings, and the choice of assay should follow current laboratory recommendations [Pregnancy-associated glycoprotein concentration in plasma and milk samples for early pregnancy diagnosis in Lacaune dairy sheep](https://pubmed.ncbi.nlm.nih.gov/25613086/). Regulatory reporting obligations vary by jurisdiction, but suspected border disease or other notifiable reproductive pathogens should be discussed with the relevant animal health authority, and international movement of animals should follow the standards of the World Organization for Animal Health [WOAH terrestrial animal health standards](https://www.woah.org/en/what-we-do/standards/codes-and-manuals/terrestrial-code-online-access/).

## Frequently Asked Questions

### How Should I Adapt the Protocol When Only a Low-Frequency Linear or Sector Transducer Is Available?

A 5 MHz transducer designed for equine rectal use was used successfully for early ovine pregnancy diagnosis in standing ewes, with each examination requiring less than one minute and producing no apparent distress. When only a 3.5 MHz sector transducer is available, image resolution at the depths typical of the ovine pregnant uterus is often adequate for detecting the fluid-filled uterine lumen and fetal heartbeat after day 30, but earlier detection becomes unreliable. Clip the wool over the caudal ventral abdomen, apply coupling gel generously, and angle the beam toward the pelvic inlet. For a 7.5 MHz linear transducer, depth penetration may limit imaging in obese ewes or those with excessive abdominal fat. In all cases, document the transducer frequency and note that sensitivity is reduced relative to a dedicated 5 MHz probe.

### What Is the Minimum Practical Investment in Time and Labor for Scanning a Commercial Flock?

A skilled operator using real-time ultrasound can examine each ewe in less than one minute when animals are restrained in a standing position. For a flock of 200 ewes, allow 3 to 4 hours including handling, drafting, and record keeping. The limiting step is usually restraint and movement of animals through the race, not the scan itself. Scanning in early gestation, before day 40, requires more time per animal because the uterine horns must be located systematically and the image interpreted with greater care. Scanning after day 45 is faster because the pregnant uterus is larger and more ventral. Plan for one person to restrain the ewe, one to scan, and one to record results and apply paint marks. This team can process 60 to 80 ewes per hour in a well-designed handling system.

### How Do I Distinguish Pregnancy from Hydrometra or Pyometra When the Findings Are Ambiguous?

Hydrometra and pyometra produce an anechoic or echogenic fluid-filled uterine lumen that can mimic the fluid accumulation of early pregnancy. The distinguishing feature is the absence of a fetus and the absence of a placentome. In pregnancy, fetal structures become visible from day 20 to 25 with a 5 MHz transducer, and placentomes appear from day 35 onward. If no fetus is identified despite a fluid-filled uterus, scan again after 7 to 10 days. A persistent fluid-filled uterus without fetal structures on two examinations separated by 10 days is unlikely to be a viable pregnancy. Border disease, caused by transmission of bovine viral diarrhea virus from persistently infected cattle, can produce fetal death and mummification, which may present as a small fluid-filled uterus without a detectable heartbeat. In such cases, the ewe will fail to lamb at the expected time.

### What Records Should I Keep for Each Scanning Session, and How Long Should They Be Retained?

Record the date, the ewe identification number, the gestational age based on breeding dates or fetal measurements, the number of fetuses detected, and the operator's confidence level for each diagnosis. Note the transducer frequency and the scanning approach used. Retain these records for at least the duration of the pregnancy plus one lambing season, as they are needed to predict lambing dates, identify ewes requiring assisted lambing, and investigate reproductive failure. For ewes scanned as not pregnant, retain the record to allow assessment of breeding management and to identify ewes for culling or rebreeding. If the flock is enrolled in a health scheme or if pregnancy data are used for genetic evaluation, follow the recording requirements of that program. The [Society for Theriogenology](https://www.therio.org/) provides guidance on reproductive record standards.

### How Should I Present a Diagnosis of Non-Pregnancy to a Producer Who Relies on Scanning for Lambing Management?

State the finding directly and give the confidence level. A negative test at day 25 has a predictive value of 80%, meaning that one in five ewes diagnosed as not pregnant may in fact be pregnant. By day 35 to 40, the negative predictive value is substantially higher. Advise the producer to re-scan any ewe that was diagnosed as not pregnant before day 35 if she does not return to oestrus. For ewes confirmed as not pregnant, discuss the likely causes, including ram fertility, timing of breeding, and early embryonic loss. Border disease should be considered when there is a pattern of early fetal loss or barren ewes, particularly if cattle persistently infected with bovine viral diarrhea virus are kept in close contact with the flock. Refer to the [MSD Veterinary Manual](https://www.msdvetmanual.com/) for a systematic approach to investigating flock-level reproductive failure.

### When Should I Recommend Laboratory-Based Pregnancy Testing Instead of or in Addition to Ultrasound?

Ultrasound is the method of choice for determining pregnancy status, fetal number, and fetal viability in sheep. Laboratory testing for pregnancy-associated glycoprotein (PAG) in plasma can detect pregnancy from day 20 onward in all pregnant ewes, and milk testing is reliable from day 28 onward. PAG concentrations in plasma are higher in multiple than in single pregnancies, but the overlap between groups limits its use for fetal number determination. Consider PAG testing when ultrasound is unavailable, when very early diagnosis is required before day 25, or when the operator lacks experience in early scanning. PAG testing cannot assess fetal viability, and a positive result may persist for some time after fetal death. Use PAG testing as an adjunct to ultrasound, not a replacement, when the clinical question is fetal number or viability.

## 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

- [Review: Large offspring syndrome in ruminants: current status and prediction during pregnancy.](https://pubmed.ncbi.nlm.nih.gov/37567678/). 2023.
- [Border disease in sheep caused by transmission of virus from cattle persistently infected with bovine virus diarrhea virus.](https://pubmed.ncbi.nlm.nih.gov/1851350/). 1991.
- [Establishment of an ELISA for measuring bovine pregnancy-associated glycoprotein in serum or milk and its application for early pregnancy detection.](https://pubmed.ncbi.nlm.nih.gov/19032429/). 2010.
- [The use of real-time ultrasound rectally for early pregnancy diagnosis in sheep.](https://pubmed.ncbi.nlm.nih.gov/16726157/). 1986.
- [The first case of a bull persistently infected with Border disease virus in New Zealand.](https://pubmed.ncbi.nlm.nih.gov/22550971/). 2012.
- [Pregnancy-associated glycoprotein (PAG) concentration in plasma and milk samples for early pregnancy diagnosis in Lacaune dairy sheep.](https://pubmed.ncbi.nlm.nih.gov/25613086/). 2015.
- [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.

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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.