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 normal pregnancy can run anywhere from about 320 to 370 days and still produce a healthy foal. Several things push the length up or down:

• Age. Older mares tend to carry a few days longer than younger mares.
• Breed. Draft breeds sometimes carry slightly longer than light riding breeds.
• Season. Mares bred in spring, their natural breeding season, tend to foal closer to 340 days. Mares bred later in the year often carry longer, which pushes the birth toward better weather the following spring.
• First-time mothers. Maiden mares (mares foaling for the first time) often run a few days past 340.

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, a veterinary ultrasound between 14 and 30 days can estimate the breeding 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. Because Mare B is a maiden mare, she may foal a few days later than the calculated date, which could shift the gap either way.

Why the timing matters

Veterinarians usually want to see a newly bred mare at set points: around day 14 to confirm pregnancy and check for twins, and again near day 30 to confirm a heartbeat. 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 most mares foal at night and the birth itself usually takes 15 to 30 minutes once active 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, two to three days before: the muscles around the tailhead 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 and not usually a cause for alarm before day 360. Past day 360, a veterinary check is a reasonable precaution to confirm the foal is developing normally.

Can twins change the gestation length? Yes. Horses rarely carry twins to term successfully, and when they do, the pregnancy is often shorter than average, with a higher risk of complications. An ultrasound around day 14 can detect twins early enough for a veterinarian to reduce one embryo if needed.

Does the breeding season affect gestation length? Yes. Mares bred in spring tend to gestate closer to 340 days. Mares bred later in the year often carry longer, which tends to delay the birth until milder weather.

Can this calculator be used for donkeys or other equids? The 340-day figure is specific to horses. Donkeys typically gestate around 365 to 370 days, and zebra gestation varies by species, 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.

Related reading

For general background on equine reproduction, veterinary organizations such as the American Association of Equine Practitioners publish guidance on mare care, gestation, and foaling.