Horse Gestation Calculator

How to use the Horse Gestation Calculator#

1. Enter the breeding date. Use the date of mating, or of insemination.
2. Check the length. Leave the average, or enter your vet’s figure for the breed.
3. Read your dates. You get the expected date, the likely window and your mare’s progress.

How long is a mare pregnant?#

The average is about 340 days, but a normal range runs from about 320 to 360 days or more. The season, the mare’s age, the foal’s sex and even the daylight length can shift it. Don’t worry about a mare going a few weeks past 340 days if she is otherwise well, but do call your vet if you’re concerned.

Getting ready for the foal#

Vets usually check for pregnancy with ultrasound about 14 to 16 days after ovulation, and again later. In the final month, mares should be moved to a clean foaling box, and many owners use a foaling alarm or camera. Signs of foaling include a full udder, waxing at the teats and restlessness.

Foaling is usually fast and can go wrong quickly, so call your vet right away if the mare strains for more than a few minutes without progress.

Why the date is a window#

Mares have the widest window of the animals on this site. Anything from 320 to 360 days is normal, and a few weeks past 340 is no cause for worry if she is otherwise well. The season, her age, the foal’s sex and the length of daylight can all shift it, so don’t plan around day 340 as a fixed date. This calculator is a planning tool and doesn’t replace veterinary advice.

A worked example#

A mare is covered on Mon 15 February 2027. Using the 340-day average, the foal is due on Fri 21 January 2028. The window runs from Sat 1 January 2028 (day 320) to Thu 10 February 2028 (day 360). Half the term is behind her by Wed 4 August 2027, and the final stretch begins on Wed 1 December 2027, which is day 289.

That is a spread of 40 days, longer than the whole gestation of a cat. It is normal, and it is the reason most studs treat any date inside it as on time.

Why breeders cover early in the year#

Many Thoroughbred and other racing registries treat every foal as a year older on 1 January in the northern hemisphere. A foal born in February is therefore competing against others that are almost a year older, and the later in the year the foal arrives, the greater the disadvantage. Breeders try to cover mares from February onward so that foaling falls early in the following year, which is why a covering date in mid-February and a foal in late January is such a common pairing.

The trade-off is that mares covered early in the season have a harder time getting in foal, because the cycle starts up slowly in the short days of winter. Long daylight shortens gestation slightly, so foals due in summer tend to arrive earlier.

The three-hour rules at foaling#

Foaling is usually quick and often happens at night, so many owners use a camera or a foaling alarm. The foal should stand within about an hour, nurse within two, and the mare should pass the placenta within three. A placenta that is still in place after three hours is an emergency call to the vet.

Plan vaccinations and worming with your vet. Boosters are normally given four to six weeks before the due date, which here is roughly Sun 12 December 2027. Move the mare to a clean, well-bedded box about a month ahead, so she builds immunity to the germs on the premises and her colostrum carries the right antibodies.

Foal heat arrives about a week after the birth, and some studs cover the mare then. Most vets suggest waiting for the second heat if the birth was difficult.

Teaching yourself to read a mare#

Every mare has her own pattern. Some always carry past 350 days, others foal at 330 each year. Keep a note of each foaling date against the covering date, and you will soon know which kind she is. Pair that with the usual signs: a full udder, wax on the teats, and a change in behaviour, and you will have a much better forecast than any average gives you.

Last reviewed October 2026 by the Calclulo team.