Gestation Calculator

Horse gestation calculator

A mare carries a foal about 340 days, but anything from 320 to 370 is normal — the widest spread of any farm animal. Enter the covering date for an estimate and the vaccination dates that come first.

340 days

Put in the date she was bred and you will get the due date, the normal window either side of it, and the dates worth writing on the calendar between now and then. The full chart is further down the page.

How far apart the breeds really are

Every dot is a breed average. The shaded band is the normal window for a single horse — which is narrower than the gap between the shortest and longest breed, and that is the part worth noticing.

Andalusian / Spanish Purebred 337average 340Thoroughbred 344

Gestation length by breed

Every figure below traces to a published study, and the ones we could not source are left out rather than guessed at. Pick a breed in the calculator and both the date and the window move with it.

BreedAverage gestationvs. horse averageNotes
Andalusian / Spanish Purebred337 days-3
Shetland pony337 days-3
Friesian338 days-2
Carthusian Spanishbred339 days-1Normal range 319–359.
Arabian340 dayseven
Haflinger pony341 days+1
Fjord pony342 days+2
Draught horse343 days+3Shorter than Thoroughbreds, despite what gets repeated.
Thoroughbred344 days+4A 16,226-mare study gives 347.0, range 306–390.

The detail, if you want it

The widest window in livestock

A 340-day average with a normal range of 320 to 370 days means a foaling date is a genuine estimate and not much more. A mare who foals at 355 days is not late. She is a mare who carries long, and she will probably do it again next year.

Individual mares are far more consistent with themselves than mares are as a group. Once you have a foaling date on a mare, her own history predicts her better than the 340-day average ever will.

Season pulls the date around

Mares covered in late winter tend to carry longer than mares bred in summer, by roughly three days for every month earlier. The studies disagree enough about it that we have not built it into the date, so treat it as something to expect rather than something to count on.

The dates that are not negotiable

Early ultrasound around day 14 is about twins, not confirmation. Twins in horses rarely end well and the window for managing them cleanly is narrow, which is why that scan is the one to book before any other.

After that: rescan around days 25 to 30 for a heartbeat, then EHV-1 vaccinations at five, seven and nine months to protect against abortion, and pre-foaling boosters about four to six weeks out so the colostrum carries the antibodies. The calculator lays all of those out as dates.

Full horse gestation chart

Every day of the year, bred to due — built to print

Find the breeding date in the left column of the right month, read across for the due date. Hit print and it comes out clean.

Bred in January
BredDue
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Bred in February
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Bred in March
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Bred in April
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Bred in May
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Bred in June
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Bred in July
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Bred in August
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Bred in September
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Bred in October
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Bred in November
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Bred in December
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Built on a 340-day gestation. Dates are month and day only, so this chart works for any year — with one catch: if the pregnancy runs through 29 February, the real due date lands one day later than the chart says. The calculator at the top of the page already accounts for that.

Questions people actually ask

How long are horses pregnant?

About 340 days, or eleven months. The normal range is wide — anywhere from about 320 to 370 days can be perfectly normal.

Why is the foaling window so wide?

Mares vary far more than cows or sows do, and the season she was bred in shifts things by a week or more on its own. A foal born at 320 days and one born at 365 can both be healthy and full term.

Individual mares are consistent, though. If you have her last two or three foaling dates, use those and ignore the 340.

Can a foal be born early and survive?

Under about 320 days a foal is genuinely premature and needs veterinary help immediately. Between 320 and 330 it depends heavily on the individual foal. Readiness for birth in horses tracks maturity rather than day count, which is why gestation length alone is a poor guide at the margins.

What are the signs a mare is close to foaling?

The udder fills over the last two to four weeks. Waxing — small beads of dried colostrum on the teats — usually shows 24 to 48 hours out, though not every mare waxes. The muscles either side of the tail slacken and the vulva lengthens.

Most mares foal at night and foal fast. Once the water breaks you should have a foal within twenty minutes to half an hour, and if it is taking longer than that, call the vet rather than waiting.

When should a mare be scanned in foal?

Around day 14 first, specifically to catch twins while they can still be managed. Then again at 25 to 30 days for a heartbeat, and often at 45 and 60 days to confirm she has held.

Where these numbers come from

Every figure on this page traces to a university extension publication or a veterinary reference. If you think one is wrong, the sources are right here so you can check it yourself.

Worth knowing

  • The width of the window is the real answer here, not the average. Anything from 320 to 370 days is normal, the largest dataset recorded live foals from 306 to 390 days, and foals past 400 days are documented. Treat the date as a rough marker and watch her instead.
  • Under 320 days counts as premature. That does not mean the foal is lost — viable foals are documented from 315 days — but it is a veterinary event rather than a routine foaling.
  • Go on her own record before anything on this page. Repeatability runs 0.36 to 0.50, and the mare accounts for about 18% of the variation against the stallion's 4%. Her last two or three foaling dates are the best predictor available — though that still leaves more than half the variation unexplained.
  • Colts run about two to four days longer than fillies. That one is consistent across four independent datasets.
  • Mares bred early in the year tend to carry longer — roughly three days for every month earlier. The evidence is genuinely mixed, so we have not built it into the date. Take it as something to expect rather than something to count on.
  • Extension sources cluster around 335–342 days and peer-reviewed Thoroughbred data around 344–347. The gap is mostly dating method: extension counts from the cover, the research counts from ultrasound-confirmed fertilisation.

Figures last checked August 2026.

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