Blood Test to Detect and Monitor Pregnancy in Mares

Veterinarians and breeders can now detect pregnancy in mares using an ELISA (enzyme-linked immunosorbent assay) test that detects estrogen molecules in a mare's blood sample and is cheaper and easier to perform than an ultrasound.

BioTracking, a company founded by Garth Sasser, PhD, a professor of animal science at the University of Idaho, recently introduced a test that can detect pregnancy in mares with high accuracy at 70 days post-breeding and two weeks before foaling. According to Jeremy Howard, sales manager at BioTracking, breeders and veterinarians can draw blood from mares and send samples to BioTracking's lab. Earlier this year, the company released a similar test that aids in tracking the pregnancy.

The specific molecules detected by the blood test are estrone sulfate, a hormone produced in high concentrations by the placenta and fetus that is present in a pregnant mare's blood and urine.

During the first month of pregnancy the blood level of estrone sulfate conjugate is very low (below 5 nanograms [ng]/mL). It rises between Days 38 and 42, due to estrogen produced by the corpus luteum (what remains of a follicle after an egg has ovulated and which produces the hormone progesteron) in the ovary. Estrone sulfate concentration in blood plasma gradually increases to a relatively high level (12 ng/mL of plasma) by Day 70. The hormone concentration continues to increase and remains significantly higher than in nonpregnant mares.

When evaluating a blood sample, veterinarians consider mares with concentrations above 8-10 ng/mL to be pregnant and those with lower levels not pregnant. Samples can be definitive for determining pregnancy any time after 70 days, but a sample taken two weeks or closer to foaling might not be reliable, since estrone sulfate levels can drop prior to foaling.

According to Howard, some broodmare owners choose to have their veterinarians perform an early ultrasound to ensure the mare is pregnant and then use the blood test to monitor the pregnancy and the fetus' health.

The fetus is the main source of estrone sulfate in the mare's blood. A normal estrone sulfate concentration indicates the fetus is healthy and viable. If plasma level of this hormone drops below normal, this could indicate the fetus is stressed or dead.

A blood sample should be collected into a "red top" vial (a blood collection tube with no anticoagulants or additives), and refrigerated until sent to one of BioTracking's laboratories. Cool packs are required during shipment. The test costs $25 to run, and BioTracking reports results via phone, e-mail, fax, or mail.

About the Author

Heather Smith Thomas

Heather Smith Thomas ranches with her husband near Salmon, Idaho, raising cattle and a few horses. She has a B.A. in English and history from University of Puget Sound (1966). She has raised and trained horses for 50 years, and has been writing freelance articles and books nearly that long, publishing 20 books and more than 9,000 articles for horse and livestock publications. Some of her books include Understanding Equine Hoof Care, The Horse Conformation Handbook, Care and Management of Horses, Storey's Guide to Raising Horses and Storey's Guide to Training Horses. Besides having her own blog,, she writes a biweekly blog at that comes out on Tuesdays.

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