Mares have been known to be irregular in their cycling, making the early determination of pregnancy difficult. Horse owners and breeding operations spend extraordinarily large amounts of money and time in breeding mares; feed, electricity, labor, water bills, barns, employees, stud fees, transportation and veterinary bills for just one breeding mare can be astronomical; thus, veterinarians and researchers are continually searching for newer and more reliable methods of diagnosing pregnancy. The costs involved often reflect the goal of the reproductive program, i.e., breeding performance or show horses tend to carry higher costs than breeding horses for pleasure riding.

Horses are known for having relatively poor reproductive performance. What this means is that in comparison to other animals, mares tend to abort their embryos far more frequently. There are many factors that can cause early embryonic loss. Underlying illness, stress, uterine infections or hormone abnormalities can all cause a mare to spontaneously abort.

Equine Gestation Basics

In the horse, the egg is fertilized in the fallopian tubes and does not enter the uterus until it about Day 6 of gestation.¹ After the embryo migrates into the uterus, it must then travel through the uterus to be recognized by the mare’s body as a viable pregnancy. If the embryo has not implanted by day 16 of gestation, the mare will usually reject the embryo and begin showing signs of estrus. If the mare’s body does recognize the presence of the embryo, then the embryo will implant itself in the wall of the uterus. The veterinarian can perform transrectal ultrasound as soon as Day 25 of gestation in order to confirm fetal viability.²

Prior to ultrasound becoming widely available, most horse owners and breeding managers relied on the fact that most mares come back into heat between 16 to 20 days after breeding if they have not yet conceived. In some areas, veterinarians used a sterile speculum to determine if the mare’s cervix was tightly closed or relaxed. Seasoned equine veterinarians can palpate a bulge in a mare’s uterus by Day 30 to 35 of gestation.^2,3

The usual intervals for checking pregnant mares are:

Day 14 to 16 – Confirmation of pregnancy and determination of twins.

Day 26 to 30 – Confirmation of fetal viability.

Day 45 – Elective examination; Should the mare aborts her pregnancy at or after this point, it is unlikely she will become pregnant again the same breeding season.

Day 60 – Elective examination.¹

Twins are far more common in Thoroughbreds than in other breeds, with around 25% to 30% of Thoroughbred mares conceiving twins. Most equine veterinarians recommend that pregnant mares should be examined for twins with transrectal ultrasound, with many checking for twins at Day 15 or 16, when the twins would be around 15 days old. Most twin sets have one larger, healthier fetus and one smaller. Almost 90% of all twins are spontaneously aborted, with almost all succumbing.² In cases where twins are detected, the veterinarian will typically abort the smaller one to ensure the viability of the larger. The mare is typically examined 48 to 72 hours afterward to confirm that the remaining embryo has survived, whether the abortion has been spontaneous or performed by the veterinarian. Prior to the advent of ultrasound, these determinations were not possible until a mare spontaneously aborted, which often threatened the viability of the surviving twin.

Estrogens and Hormone Therapy

Rectal palpation is a reliable and cost-effective method of determining pregnancy in mares, however, it is not accurate at less than 30 days of gestation. It is also unreliable for the early diagnosis of twins. There are several hormone-based tests that have been developed since the original hemagglutination inhibition test (Mare Immunopregnancy or MIP test).²

Estrone Sulfate (E1S) is an estrogen conjugate that serves as a stable circulating reservoir of estrogen. In humans, levels of E1S are the highest among the estrogens in postmenopausal women. Recently, studies demonstrated the occurrence of feminized male fish at sites impacted by hormonal contaminants (arising from human and animal wastes). Those receiving the greatest attention in terms of research and monitoring have been 17β-estradiol (β-E2) and 17α-ethinylestradiol, due both to their prevalence in the environment and strong estrogenic potency. Estrone was also found at high concentrations in surface waters but was generally of lesser concern due to its relatively lower affinity for vertebrate estrogen receptors.⁴

In veterinary medicine, estrone sulfate has been widely used in managing performance horses and for the determination of pregnancy in mares. Estrone sulfate concentrations in the blood have also been used in a limited manner as an indicator of fetal demise (abortion) in horses.³

Starting from about 60 days of gestation, assays for serum/plasma estrone sulfate (ES) can be used to establish the presence of pregnancy in the mare. The estrone sulfate test is very accurate for diagnosing pregnancy in mares from day 90 to term.^3,4  Estrone sulfate is produced by the fetal-placental unit and is therefore a good indicator of fetal condition. Estrone is occasionally administered to performance horses as a precaution against exercise-induced pulmonary hemorrhage (EIPH).

Estrogens are also used by veterinarians for hormone replacement therapy and to treat horses exhibiting clinical signs of intermittent upward patellar fixation (IUPF) or Proximal Patellar Hesitation (PPH).³ These protocols came about based on the idea that estrogen increases the tension the horse’s ligaments, thereby increasing the tone of the distal patellar ligaments and preventing the patella from interfering with stifle function.⁴

Estrogens used in these applications are available in two primary forms: Estrone sulfate and Estradiol cypionate. Both carry advantages and disadvantages which are best evaluated by the veterinarian. Estrone sulfate is a water-based suspension that is easily administered by the veterinarian, owner or manager and easily absorbed in the horse's muscle. Its disadvantages are that its water-solubility results in a shorter duration of action. Estradiol cypionate is an oil-based product that is more slowly absorbed into the horse's system and has a long-lasting effect. A main disadvantage lies in the fact that the prepared suspensions can be very thick and difficult to administer.

Where to buy Estrone Sulfate

Estrone sulfate is available in the U.S. through several pharmaceutical manufacturers and through veterinary custom compounding companies. ESTRONE SULFATE 5 MG/ML by NexGen Pharmaceuticals is an excellent product for managing performance horses and for the determination of pregnancy in mares.

Please consult your veterinarian prior to beginning any treatment regimen.

FOR RX ONLY: A valid prescription from a licensed veterinarian is required for dispensing this medication.

¹Allen, W.R. (2005). Maternal recognition and maintenance of pregnancy in the mare. Anim Reprod, 2:209-223.

²Bergfelt, D.R., Woods, J.A., & Ginther, O.J. (1992). Role of the embryonic vesicle and progesterone in embryonic loss in mares. J Reprod Fertil. 95:339-347.

³Kasman LH, Hughes JP, Stabenfeldt GH, Starr MD, Lasley BL. Estrone sulfate concentrations as an indicator of fetal demise in horses. Am J Vet Res. 1988 Feb;49(2):184-7. PMID: 2831761.

⁴Ankley, G., et. al. Re-evaluating the Significance of Estrone as an Environmental Estrogen. Environmental Science & Technology 2017 51 (8), 4705-4713.