Horse Pregnancy Calculator - Track Your Mare's Foaling Date

What is a horse pregnancy calculator?

A horse pregnancy calculator estimates a mare's foaling date from her breeding date. It adds the average equine gestation length, 340 days, to the date she was bred, and shows which trimester she is in and how many days remain.

How long is a horse pregnant?

The average gestation length in horses is 340 days, close to eleven months. A University of Nebraska guide gives an average of 335 to 340 days and a normal range of 320 to 370 days. Several things push the length up or down:

• Foal sex. Colts tend to be carried 2 to 7 days longer than fillies.
• Body condition. Thin mares tend to carry slightly longer than mares in good condition.
• Season. Mares that foal in spring and summer, when days are long, tend to have shorter pregnancies than mares that foal in January or February.

Because of this spread, the calculated date is a planning target, not a guarantee. Breeders usually treat it as the middle of a window rather than a deadline.

Foaling date formula

The expected foaling date is the breeding date plus 340 days:

$\text{Foaling Date} = \text{Breeding Date} + 340 \text{ days}$

Example. A mare is bred on March 15, 2023. Add 340 days: her expected foaling date is February 18, 2024.

Trimesters of horse pregnancy

The 340-day pregnancy splits into three trimesters, counting from day 0 (the breeding date):

To find the current trimester, count the whole days since breeding and compare against these cutoffs: 114 days or fewer is the first trimester, up to 226 days is the second, and anything after that is the third.

Example. A mare bred on March 15, 2023 reaches day 115 of her pregnancy on July 8, 2023, so her second trimester starts that day. She reaches day 227 on October 28, 2023, when her third trimester begins.

How to use the horse pregnancy calculator

1. Enter the breeding date. For hand breeding or artificial insemination, use the known service date. For live cover over several days, use the last date. For pasture breeding with an unknown date, an early veterinary ultrasound can estimate the conception date from the size of the embryo.
2. Read the results: the expected foaling date, the current trimester, and the number of days remaining.
3. Check back periodically as the pregnancy moves forward, and compare the calculated milestones with what the mare actually shows.

Worked example: two mares in the same season

The two expected foaling dates are 26 days apart, so a breeder with both mares should expect back-to-back foaling watches rather than a single event. Either mare may foal days or weeks away from her calculated date, which could shift the gap either way.

Why the timing matters

An ultrasound check is accurate from about 14 days after breeding, and the heartbeat should be visible by day 24. Veterinarians therefore often see a newly bred mare around day 14 to confirm pregnancy and check for twins, and again later to confirm a heartbeat. The calculator marks these checks at day 14 and day 30. Feed is often adjusted going into the third trimester, when the fetus grows fastest. Foaling stalls are typically cleaned and stocked with supplies a few weeks before the expected date, and someone needs to be available to check on the mare as the date approaches, since 75 to 85 percent of foals are born between 6 p.m. and 6 a.m. Delivery usually takes 10 to 30 minutes once hard labor starts.

Signs a mare is close to foaling

• Udder development, two to six weeks before foaling: the mammary glands fill and enlarge.
• Waxing, one to four days before: dried colostrum forms small waxy beads on the teats.
• Muscle relaxation, during the last one to two weeks: the muscles around the tailhead and pelvis gradually soften.
• Restlessness, in the final hours: pawing, sweating, and repeated lying down and standing up.

Not every mare shows every sign, and the order can vary, so none of these alone confirms an exact foaling time.

Frequently asked questions

How accurate is a horse pregnancy calculator? The 340-day estimate is a useful planning date, not an exact prediction. Individual mares commonly foal anywhere from about 320 to 370 days and are still considered normal.

What if my mare passes day 340 without foaling? This is common. Pregnancies as long as 365 days are not unusual, but longer pregnancies can be a sign of foaling problems, for example in mares eating fescue hay or pasture infected with an endophyte fungus. A veterinary check is a reasonable precaution for a mare well past her date.

Can twins change the gestation length? Yes. Twin pregnancies often end early, because twins are linked to late-term abortion when the uterus becomes crowded. An ultrasound around day 14 can detect twins early enough for a veterinarian to reduce one embryo, which is best done before day 16.

Does the breeding season affect gestation length? Yes. Mares that foal in spring and summer, when days are long, tend to have shorter pregnancies than mares that foal in January or February.

Can this calculator be used for donkeys or other equids? The 340-day figure is specific to horses. Donkeys gestate for about 365 days, so results for other equids need a different starting number.

How is the current trimester determined? By counting whole days from the breeding date. Days 0 through 114 fall in the first trimester, days 115 through 226 in the second, and day 227 onward in the third.

References

1. Ontario Ministry of Agriculture, Food and Agribusiness. Foaling and predicting foaling time. Foaling and predicting foaling time — average mare gestation of 340 days.
2. University of Nebraska–Lincoln Extension. The Foaling Mare (NebGuide G1874). The Foaling Mare — average 335 to 340 days, range 320 to 370; colts carried 2 to 7 days longer; thin mares and January–February foaling lengthen gestation; udder development 2 to 6 weeks before foaling; pelvic relaxation in the last 7 to 14 days; 75 to 85 percent of foals born between 6 p.m. and 6 a.m.; stage two labor 10 to 30 minutes.
3. Oklahoma State University Extension. Foaling Management and Care of the Nursing Foal (AFS-3985). AFS-3985 (PDF) — waxing 1 to 4 days before foaling; gestations of 365 days not uncommon, longer ones a possible sign of problems such as fescue toxicity.
4. University of Georgia Cooperative Extension. Understanding Gestation in the Mare and the Potential for Problems (B1461). Bulletin B1461 — ultrasound accurate from about 14 days; heartbeat by day 24; twins linked to late-term abortion; reduction best before day 16.
5. Merck Veterinary Manual. Approximate Gestation Periods. Gestation periods table — donkey gestation of 365 days.