Early Embryonic Loss in Horses: Causes, Diagnosis, and Prevention

Early embryonic loss is one of the most common reproductive problems in horses and a major cause of reduced breeding efficiency. It refers to the death and resorption or expulsion of the embryo before 40 days of gestation, after pregnancy has been confirmed.

Although frustrating for breeders, early embryonic loss is not uncommon, estimates suggest that up to 10–15 percent of pregnancies fail during this early stage, even under ideal conditions. Understanding the causes, risk factors, and management strategies can help improve conception rates and reproductive success in mares.

What Is Early Embryonic Loss

In a normal pregnancy, fertilisation occurs in the oviduct, and the embryo travels to the uterus around day six after ovulation. By day 14, the embryo should be visible on ultrasound as a small, fluid-filled sphere. Between days 16 and 40, the developing conceptus becomes more firmly attached to the uterine lining and begins to form the early placenta.

Early embryonic loss occurs when this process is disrupted and the embryo dies before the placenta is fully established. The mare may then return to oestrus after a delayed or irregular cycle, or she may appear to maintain pregnancy for a short time before reabsorbing the embryo.

Recognising Early Embryonic Loss

One of the challenges of early embryonic loss is that it often happens silently, without visible signs. Some mares may show subtle changes, such as:

• Failure to return to heat when expected after breeding, then coming back into season several weeks later
• Irregular or prolonged oestrous cycles
• Mild uterine discharge or changes in tone detected on examination
• Unexpectedly small or absent embryo on ultrasound at recheck

Because these signs can be subtle or easily missed, ultrasound examinations at regular intervals are essential for accurate pregnancy monitoring.

Causes of Early Embryonic Loss

There is rarely a single cause for early embryonic loss; instead, it results from a combination of factors affecting the embryo, uterus, or mare’s overall health.

1. Chromosomal Abnormalities

Many early embryonic losses occur because of genetic or chromosomal defects that prevent normal development. These abnormalities often arise during fertilisation and are beyond the control of the breeder or veterinarian.

2. Uterine or Endometrial Problems

A healthy uterine environment is essential for embryo survival. Inflammation (endometritis), scarring (endometrial fibrosis), or fluid accumulation can interfere with embryo attachment and nourishment. Mares with a history of uterine infections or poor uterine clearance are at higher risk.

3. Hormonal Imbalances

Inadequate production of progesterone (the hormone that maintains pregnancy) can lead to loss of the embryo. This may result from problems with the corpus luteum, the structure on the ovary that produces progesterone. Some mares benefit from hormone supplementation (such as altrenogest) during early pregnancy if this is suspected.

4. Twin Pregnancies

If two embryos develop, one or both are likely to be lost. Mares are not designed to carry twins, and competition for uterine space and nutrients usually results in early loss. Early ultrasound is critical to detect and reduce twin pregnancies before day 17.

5. Infection and Inflammation

Bacterial contamination of the uterus, often introduced during breeding or associated with poor vulvar conformation, can cause endometritis and disrupt the early pregnancy environment. Viral infections such as equine herpesvirus (EHV-1) are less common at this stage but can also contribute.

6. Stress and Systemic Illness

Transport, environmental changes, high fever, or severe illness in the mare can cause hormonal disruption or inflammation that leads to embryonic loss. Stress-related prostaglandin release may prematurely terminate the pregnancy.

7. Nutritional Deficiencies

Poor body condition or deficiencies in essential vitamins and minerals, such as vitamin E, selenium, or copper, may affect embryo survival and uterine health. Balanced nutrition is particularly important during breeding and early pregnancy.

8. Age of the Mare

Older mares have a higher risk of early embryonic loss due to reduced uterine quality, less efficient ovulation, and hormonal irregularities. The risk increases significantly after about 15 years of age.

9. Stallion or Semen Factors

Poor semen quality, low motility, or DNA damage in sperm can contribute to early embryo death. Improper semen handling or timing of insemination can also play a role in artificial breeding programs.

Diagnosis

Diagnosing early embryonic loss requires careful veterinary evaluation and timed reproductive monitoring.

• Ultrasound scans at 14–16 days post-ovulation confirm pregnancy and detect twins. Follow-up scans at 25–30 days ensure the embryo continues to develop and a heartbeat is present.
• If the embryo is absent, small, or irregular, loss may have occurred.
• Uterine culture and cytology can identify infection or inflammation.
• Hormone assays may be used to assess progesterone levels.
• A complete breeding and management history can help identify underlying factors.

Treatment and Management

Once early embryonic loss has occurred, treatment focuses on correcting any underlying causes and optimising conditions for the next breeding cycle.

1. Treat uterine infections with appropriate antibiotics or uterine lavage.
2. Supplement progesterone if hormonal deficiency is suspected, using altrenogest or injectable progesterone.
3. Perform early ultrasound checks in the next cycle to monitor ovulation and uterine health.
4. Adjust breeding timing to ensure insemination aligns with ovulation.
5. Improve mare condition through proper nutrition and exercise before breeding again.
6. Minimise stress by avoiding transport or major changes during early pregnancy.

Most mares can be rebred successfully in the following cycle once the cause is addressed.

Prevention

Preventing early embryonic loss relies on attention to detail in breeding management, mare health, and environmental stability.

Pre-Breeding Preparation

• Conduct a full reproductive exam and uterine culture before breeding.
• Correct conformational issues (such as poor vulval seal or pneumovagina) if necessary.
• Ensure the mare is in good body condition and up to date with vaccinations and deworming.

During Breeding

• Use high-quality, properly handled semen.
• Time breeding accurately with ovulation.
• Maintain strict hygiene during insemination or live cover.

Post-Breeding and Early Pregnancy Care

• Confirm pregnancy with ultrasound at 14–16 days, then recheck at 25–30 days.
• Avoid unnecessary stress, travel, or medication during early pregnancy.
• Provide consistent turnout, diet, and water supply.
• Supplement progesterone in mares with a known history of deficiency or early loss.
• Monitor older mares more closely for uterine health and hormonal stability.

Prognosis and Future Fertility

Most mares that experience early embryonic loss can conceive again and carry a healthy foal in subsequent cycles. The prognosis is excellent when the cause is identified and corrected. Chronic uterine infection or severe endometrial degeneration, however, can reduce long-term fertility. Regular reproductive assessment helps guide realistic breeding expectations.

Conclusion

Early embryonic loss in horses is a multifactorial problem that can stem from genetic, uterine, hormonal, or environmental causes. While some cases are unavoidable, many can be prevented through careful management, veterinary oversight, and proper breeding practices.

By monitoring pregnancies closely, maintaining a healthy uterine environment, and minimising stress, breeders can improve conception rates and reduce the risk of early pregnancy failure. Understanding the underlying factors is key to ensuring successful breeding outcomes and healthy foals.