Hormonal Fluctuations in Pregnant Horses and Their Effects on Reproductive Success

For breeders, veterinarians, and horse owners alike, understanding the intricate web of hormonal changes during equine pregnancy is vital for achieving optimal reproductive outcomes. Pregnancy in mares is not simply a matter of conception and gestation—it’s a complex physiological process, driven and regulated by hormones that interact delicately to sustain the embryo, prepare the uterus, and support fetal development.

In both practical fieldwork and academic research from around the world, hormonal fluctuations in pregnant mares have been closely linked with reproductive efficiency, early embryonic loss, placental function, and even foal viability. ในบทความนี้, we explore how these hormonal shifts work, what they mean for equine reproduction, and how veterinary professionals can use this knowledge to improve reproductive success.

Understanding the Hormonal Landscape of Equine Pregnancy

Pregnancy in the horse typically lasts about 340 วัน, though this can vary by a few weeks depending on breed, individual differences, and environmental factors. Throughout this period, several key hormones orchestrate changes in the mare’s body to maintain the pregnancy and support fetal growth.

The primary hormones involved are:

• Progesterone

• Estrogens (primarily estrone sulfate)

• Equine Chorionic Gonadotropin (eCG)

• Relaxin

• Oxytocin (in late pregnancy)

Each hormone has specific functions and appears in different quantities at various stages of gestation.

Early Pregnancy: The Crucial Role of Progesterone

In the early weeks of pregnancy, progesterone, secreted by the corpus luteum (CL), plays a critical role. It prepares the endometrium for implantation, suppresses estrus behavior, and ensures a suitable environment for embryo survival.

In mares, the CL forms after ovulation and remains the primary source of progesterone for about 35–40 days. By this point, the embryo must signal its presence effectively to prevent luteolysis (regression of the CL) and maintain progesterone levels. This maternal recognition of pregnancy is still not entirely understood in horses, but it is known to involve the movement of the embryo throughout the uterus.

Foreign researchers, such as Ginther (2018), emphasized that failure of proper embryo signaling can result in early embryonic loss—accounting for a substantial number of reproductive failures in sport horse and breeding operations.

Mid-Pregnancy: The Emergence of eCG and Estrogens

Around day 35–40 of gestation, equine chorionic gonadotropin (eCG) begins to be secreted by the endometrial cups—specialized structures unique to equine pregnancy. eCG stimulates the formation of supplementary corpora lutea, which produce additional progesterone, effectively reinforcing hormonal support for the pregnancy until the placenta can take over.

This is also the period when estrogen levels begin to rise. Estrogens, especially estrone sulfate, are produced by the fetoplacental unit and serve as indicators of fetal viability. By day 60, estrone sulfate is detectable in maternal blood and remains elevated through most of gestation.

Studies from European breeding farms use estrone sulfate testing as a reliable indicator of fetal health. In cases of fetal death or placental dysfunction, estrogen levels drop significantly, making hormone monitoring an essential diagnostic tool.

Late Pregnancy: Relaxin and Prepartum Changes

During the final trimester, relaxin is secreted by the placenta. It softens the cervix and pelvic ligaments in preparation for parturition. Simultaneously, oxytocin levels remain low until labor begins, when its rapid surge triggers uterine contractions.

Some American equine clinics have adopted the practice of hormone tracking, รวมถึง relaxin ระดับ, to predict parturition windows more accurately in mares with high economic or genetic value. This is especially important when veterinary intervention may be needed due to complications like dystocia.

Hormonal Imbalances and Their Impact on Reproductive Success

Even minor disturbances in the hormonal balance of pregnant mares can lead to:

• Early Embryonic Loss (EEL)

• Abortion in Mid-Gestation

• Poor Placental Development

• Delayed Parturition or Dystocia

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Early Embryonic Loss

A significant portion of EEL occurs within the first 40 วัน, often linked to insufficient progesterone levels or inadequate eCG response. In these cases, supplementary progesterone therapy is commonly administered in the field.

Foreign veterinary practices, particularly in the UK and Australia, report using Regumate (altrenogest) to sustain pregnancy in mares with hormonal deficiencies or recurrent pregnancy loss. อย่างไรก็ตาม, improper or prolonged use can suppress endogenous hormone production, requiring careful veterinary oversight.

Placental Insufficiency

Estrogens produced by the fetus and placenta serve as key markers for fetal well-being. Inadequate estrogen levels can signal poor placental development, which may result in growth-restricted or non-viable foals.

Monitoring Hormones: What Can Be Measured?

Thanks to advancements in equine reproductive technology, several hormonal tests are now widely used in international veterinary practice:

• Progesterone assay: Used to detect luteal function and identify mares at risk of EEL.

• Estrone sulfate testing: Confirms fetal viability after day 60.

• eCG levels: Evaluated in mares suspected of abnormal luteal development or twin pregnancies.

• Relaxin assays: Occasionally used for late-pregnancy monitoring.

In Europe and North America, stall-side kits and lab-based testing are both common, depending on the infrastructure of the breeding program.

Supporting Hormonal Balance Through Management

Beyond medical interventions, successful breeding operations understand the importance of management-based support for hormonal stability. Some effective strategies include:

• Minimizing stress: Mares are sensitive to stress, which can suppress progesterone secretion. Proper handling, nutrition, and quiet environments are key.

• Nutritional support: Balanced diets with adequate fats and micronutrients, particularly selenium and vitamin E, help maintain hormonal health and placental function.

• Infection control: Uterine infections are known to disrupt early pregnancy hormones. Routine reproductive health screenings are critical.

Australian breeders emphasize that environmental consistency, especially during the first 60 days of pregnancy, correlates strongly with successful outcomes.

Hormone Tracking and Reproductive Technology

Newer tools like ultrasound imaging (especially Doppler modes) complement hormone testing by providing visual confirmation of fetal heartbeats, placental attachment, and blood flow. When combined, hormone data and ultrasound form a robust basis for reproductive decision-making.

While the article does not focus on products, devices like portable veterinary ultrasounds are now widely used by reproductive veterinarians globally, ensuring hormone-related changes are always interpreted in context.

International Perspectives: Breeding for Success

In regions like Kentucky, Ireland, and Germany—home to many of the world’s leading equine breeding programs—hormonal monitoring is a standard part of prenatal care. These regions combine traditional knowledge with science-backed practices, setting benchmarks for reproductive efficiency.

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• Irish breeders often use estrone sulfate testing in combination with fetal sexing via ultrasound to make informed decisions about sales and mating plans.

• American clinics routinely check progesterone in embryo transfer recipient mares to optimize success rates.

• Japanese breeders, known for precision, integrate hormone assays with deep ultrasound evaluations for early twin reduction decisions.

บทสรุป

Understanding and managing hormonal fluctuations in pregnant mares is essential for reproductive success. From early embryo support via progesterone, to mid-gestation estrogen tracking, and late-pregnancy relaxin monitoring, each hormone tells a story about the pregnancy’s progress.

Equine reproduction is no longer left to chance. With global veterinary knowledge, advanced testing tools, and proper management, breeders today can dramatically improve pregnancy retention and foal viability. By appreciating the hormonal rhythm of equine gestation, we not only protect mare and foal but also preserve the genetics and future of every breeding program.

References

• Ginther, O. J. (2018). Reproductive Biology of the Mare: Basic and Applied Aspects. 3rd ed. Equiservices Publishing.

• Allen, W. R. (2005). “Endocrine changes during early pregnancy in the mare.” Reproduction in Domestic Animals, 40(3), 205–210. https://onlinelibrary.wiley.com/doi/10.1111/j.1439-0531.2005.00688.x

• McCue, P. M. (2009). “Fetal Monitoring in the Mare.” AAEP Proceedings, 55, 246–251. https://aaep.org/proceedings

• Aurich, C. (2011). “Reproductive cycles of horses.” Animal Reproduction Science, 124(3–4), 220–228. https://doi.org/10.1016/j.anireprosci.2010.08.016

• Woods, G. L., & Ginther, O. J. (1985). “Interaction of conceptus and endometrium in mares.” Journal of Reproduction and Fertility Supplement, 33, 115–128. https://www.jstor.org/stable/10.2307/26177938