Researchers are always looking for new ways to detect failed pregnancies, reproductive problems, or pregnancy loss in mares. Some of those indicators are tiny little molecules that we know better as hormones.

At the 2016 American Association of Equine Practitioners Convention, held Dec. 3-7 in Orlando, Florida, Alejandro Esteller-Vico, PhD, of the University of Kentucky Gluck Equine Research Center’s Equine Reproduction Laboratory, reviewed how veterinarians can use basic endocrine (hormone) tests to diagnose reproductive problems in mares. He discussed three useful hormones and groups of hormones: anti-Mullerian hormone, estrogen, and progestogens.

Anti-Mullerian Hormone (AMH)

Granulosa cells in ovarian follicles produce AMH, a measure of reproductive aging and an indication of the number of viable follicles a mare has in reserve. This hormone is also a marker for granulosa cell tumors (ovarian tumors), which could impair reproduction. Very low AMH concentrations could mean that a mare does not have many, if any, viable follicles left.


Estrogens are a group of hormones—the primary female sex hormones—synthesized by the ovary and placenta that serve as biomarkers for pregnancies. One form of estrogen—estrone sulfate (ES)—begins to increase around Days 35-40 of gestation and is a good indicator of pregnancy status, Esteller-Vico said. He cautioned that ES concentrations tend to be higher in Miniature Horses than in their larger counterparts; pregnancy might not be detectable by measuring ES until approximately 80 days of gestation in Minis. Other estrogens, such estradiol or estradiol sulfate, can be used during late gestation as biomarkers for placentitis (infection of the placenta), he added.


Progesterone, one of the most well-studied progestogens, plays a role in maintaining pregnancy. The corpus luteum (CL, a structure that develops in the ovary on the site where an ovum is released) secretes progesterone. Immediately after ovulation, a mare’s progesterone concentrations start to increase. In the absence of pregnancy, the CL degrades and progesterone concentrations drop. However, if an embryo is present, the CL’s lifespan is extended and progesterone concentrations continue to increase until 60-120 days of gestation. Progesterone concentrations can be measured to determine if there is enough hormone present to sustain pregnancy. Progestogens also can also be used during late gestations as biomarkers for placentitis, Esteller-Vico said.

Take-Home Message

Careful interpretation of hormone concentrations can help diagnose reproductive problems, determine pregnancy status, and indicate the optimum time for conception. This information is useful for maintaining the mare’s well-being and, of course, for the production and development of a healthy foal.