A mare is pregnant for about 340 days roughly 11 months though a normal pregnancy can run anywhere from 320 to 370 days (eXtension Horses). Enter your mare’s breeding date in the calculator above and it estimates her foaling date, her most-likely window, and the key management dates in between.
The number the calculator uses
The tool adds 340 days to the breeding (cover) date for a full-size horse. That figure is the average reported by university extension programs: the University of Nebraska Extension and eXtension Horses both put the average at 335–340 days. Because individual mares vary, the calculator also shows a most-likely window of roughly 330–345 days and the full normal range of 320–370 days. For ponies and miniature horses — which tend to foal a little earlier — choose the pony/mini option, which uses about 330 days.
How long is a mare’s gestation period?
The short answer: about 11 months. The practical answer is that mares are famously variable. Kentucky Equine Research notes that a spread of 320 to 370 days is completely within normal limits, and some references extend the outer edge further still. Foals delivered before about 326 days are generally considered premature and need veterinary attention. Season and foal sex nudge the number too: mares bred to foal in late spring and summer often carry slightly shorter, and colts are typically carried a couple of days longer than fillies. Treat any single due date as an estimate, not a deadline.
Mare gestation chart by type
The average gestation length shifts a little with body type. This chart shows the figures the calculator uses and the natural range around each.
| Type | Average gestation | Typical range | In months |
|---|---|---|---|
| Full-size horse (Thoroughbred, Quarter Horse, Arabian, Warmblood) | ~340 days | 320–370 days | ≈11 months |
| Draft breeds | ~340–345 days | 320–370 days | ≈11 months |
| Pony | ~330–335 days | 315–360 days | ≈11 months |
| Miniature horse | ~330 days | Viable from ~300 days | ≈10.5–11 months |
Ponies and minis, on average, carry shorter than full-size horses, which is why a dedicated miniature mare gestation calculator setting is useful — but every mare tends to repeat her own pattern year to year, so her own records are the best predictor.
How to use the mare gestation calculator
- Enter the breeding or cover date (use the last cover date if she was covered more than once).
- Choose horse or pony / miniature.
- Read the estimated foaling date, the most-likely window, and the full normal range.
- Use the timeline and management dates to plan ultrasounds, vaccinations, and the start of foaling watch.
- Tap Print / save to keep a copy in your foaling records.
Mare Gestation Calculator
Enter the breeding date to estimate your mare's foaling date and window.
Enter a breeding date above.
Mare gestation chart & management dates
| Stage / milestone | Days from breeding | Estimated date |
|---|
Mare gestation stages: what happens when
Equine pregnancy is usually split into three trimesters. According to Mad Barn, the first trimester runs from day 0 to about day 114 (implantation and organ formation), the second from day 115 to 226 (rapid growth), and the third from day 227 to foaling (weight gain and final development). Pregnancy is typically confirmed by ultrasound around days 14–16, when the vet also checks for twins; a heartbeat can usually be seen by about day 25–30, and fetal sexing is possible from roughly day 60–90. The calculator’s milestone list turns these into real calendar dates for your mare.
Frequently asked questions
How long is a horse’s gestation period?
About 340 days on average — close to 11 months — with a normal range of roughly 320 to 370 days (Chewy, DVM-reviewed).
How many months is a horse pregnant?
Around 11 months. Because gestation averages 340 days, a mare bred in May will typically foal the following April.
What counts as early or overdue?
Foals born before about 326 days are generally considered premature, while carrying past 370 days is unusual but not automatically dangerous. Anything outside the normal range is a reason to call your vet.
Is a miniature mare’s gestation different?
Slightly. Minis and ponies tend to foal a little earlier — often around 330 days — and can produce viable foals from roughly 300 days, so use the pony/mini setting and watch them closely toward the end.
How do I calculate my mare’s due date?
Add 340 days to the breeding date (330 for a pony or mini). The calculator above does this for you and also gives the surrounding window and management calendar.
When should I start foaling watch?
Udder development often begins 2–6 weeks before foaling, with “waxing” of the teats 1–2 days before. The calculator flags a suggested foaling-watch start date about a month before the due date.
Sources
- eXtension Horses — Management of the Pregnant Mare: horses.extension.org
- University of Nebraska Extension — The Foaling Mare: extensionpubs.unl.edu
- Kentucky Equine Research — Gestation in Mares: ker.com
- Mad Barn — Gestation in Horses: madbarn.com
- Chewy (DVM-reviewed) — How Long Are Horses Pregnant: chewy.com
Reviewed for accuracy against University of Nebraska Extension, eXtension Horses, and Kentucky Equine Research. Last updated: July 12, 2026.
Related BarnCalc tools: visit the horse hub, or try the horse weight calculator and horse feed calculator.