Impending Arrival

Determining when a mare will foal is art and science, but there are tests to help you pinpoint the due date.

Gestation in mares is estimated to last about 340 days, but this is just an average, since mares often foal as much as three weeks earlier or later than this standard time. Mares are notoriously unpredictable, and this is why horse breeders often try to figure out ways to more accurately pinpoint the time of foaling so they can be present at the birth.

Pete Sheerin, DVM, Dipl. ACT (reproduction specialist), of Rood & Riddle Equine Hospital in Lexington, Ky., says typically breeders figure an estimated foaling date using the mare's last ovulation or breeding date. This just gives a starting point; breeders need to be aware that the mare could foal several weeks ahead of the projected due date. When examining the mare for signs of impending foaling, things they generally look for include udder development, relaxation of the vulva, loosening of the ligaments on each side of the tailhead, and waxing at the ends of the teats.

These changes might take place within just a few days of foaling or transpire over several weeks. A mare can bag up three to four weeks before foaling and might even leak milk for several days before she actually foals. "This is the frustrating thing about foaling one or two mares at home and trying to watch them," says Sheerin. "The important thing is to know your mare. If it's a mare you've foaled out before, she will usually have a pattern. Length of gestation in an individual mare is often similar with each subsequent foaling.

"Maiden mares are tougher to predict, not only because of the variability of when they might foal, but they also may not build an udder as quickly as a mare that's had foals before," cautions Sheerin. A maiden mare might suddenly bag up and foal without much warning, and some might not produce milk until after they foal.

Testing For Readiness

"One thing we often use as an aid in prediction is testing mammary secretions," he adds. "There are several commercial test kits available for checking the mare's milk. In reality, they are more accurate in telling you when the mare is not ready to foal, than for pinpointing when she will foal."

Ahmed Tibary DVM. PhD, a theriogenology professor at Washington State University, says most commercial tests used for predicting foaling are not 100% accurate. "The kits are based on determining changes in electrolytes, particularly calcium, in the pre-colostrum secretions," says Tibary. "The science behind these tests is very good, but the tests themselves are not as reliable as if someone tested the secretions with a more precise laboratory technique that can measure several electrolytes at the same time (calcium, magnesium, sodium, and potassium)."

However, the test can give you a heads-up. "Most will give color change indicators, depending on what the calcium concentration is in the secretion, and a percent chance or probability that the mare will foal within a 24-hour period, for instance," says Tibary. "If the calcium carbonate concentration is greater than 200 parts per million (40 milligrams per deciliter), research has shown that a mare with this spike in calcium has about a 50% chance of foaling within 24 hours, 85% chance of foaling within 48 hours, and up to 95% chance of foaling within 72 hours."

Sheerin recommends using a commercial test kit or taking a sample of the udder secretion to a lab to measure calcium and the other electrolytes. This is helpful for determining when the foal is ready for birth if a mare must be induced.

A small amount of milk is taken from a teat, squeezed into a clean container, and the secretion applied to the test strip. The strip will change colors when calcium reaches a certain level, which means the mare is getting close to foaling.

"The ideal time to take the sample from the mare is in the evening," says Sheerin. "This gives you a more accurate result. I can't explain the physiology behind this, but it may have something to do with the fact most mares foal at night, so we start to see the changes then. From my experience, samples taken in the morning or through the day seem to be less predictive than the ones taken in the early evening."

If Tibary is concerned about a mare and wants to be present during foaling, he uses the tests, and once she starts to show a rising calcium level, he utilizes a monitoring system such as the Foalert. This incorporates a device stitched to the vulva lips. When the lips spread apart during early labor, the device sends an electronic signal to an alarm system in the barn, a pager, or a cell phone. Or you can use a closed- circuit TV monitor to watch the mare from your house. Some people set up a system whereby they can monitor the mare on the Internet from wherever they might be.

The milk test kits can give you a clue about foaling time, so you don't have to watch the mare as closely before she foals. But you must also take into consideration any problems the mare might have.

"If she has a placentitis, for instance, or early lactation, these tests are not very predictive," explains Tibary. "Any hormonal changes that would cause her to bag up early would mean the tests would not be accurate. The tests are only good if the pregnancy (and development of the mammary gland and secretions) is normal. It would otherwise give a false reading because the mare is already producing milk due to the change in her hormonal pattern and placental physiology.

Nope, Not Yet!

"Another complication I've observed is changing the mare's environment just before foaling," continues Tibary. "When mares are moved to foal in a hospital, for example, the test may have shown a positive prediction and high probability for foaling within 24 hours, but the mare may delay parturition for a few days. Even though she is prepared to foal, there are other external factors that may affect her foaling. She may need to become more adjusted to the new environment."

There can be other complications, as when mares are on fescue pasture. "Those mares are agalactic (producing no milk at all), yet are also prone to abnormally long pregnancies," says Tibary.

The test kits can be very helpful, however, if everything is normal. "I've used them when working for large stud farms to get a heads-up on mares you want to watch more closely, particularly the high-risk mares that may need some assistance," he says. "It helps you decide when to put the foal alert device on the mare, or to open up a mare that's been sutured with a Caslick's. If you have a lot of mares foaling and don't have enough stall space for all of them, the tests enable you to manage the mares and decide which ones need to be moved into the foaling stalls.

Take-Home Message

Predicting when a mare will foal is as much art as science. There are tests and signs that can help you narrow the time when the mare might foal, but other factors must be considered. Work with a veterinarian and an experienced foaling attendant if this is your first foal. Then enjoy the miracle!

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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