Quick answer: A mare is pregnant for about 340 days, or roughly 11 months. The normal range is wide, from about 320 to 370 days. Enter the last breeding date in this mare gestation period calculator to get her foaling date and the full window.
How long are horses pregnant?
The average horse gestation period is 335 to 340 days. A healthy pregnancy can be as short as about 320 days or as long as 370. Pregnancies of up to 365 days are not unusual. A donkey pregnancy runs about three weeks longer still.
That wide range is why the old “11 months” rule often catches owners out. Many mares go longer than 11 months, so plan for the full window rather than one date.
Mare gestation and foaling date table
Like a mare due date calculator on paper, the table below uses 340 days from the last breeding. If you are still planning that breeding, our mare in heat guide shows when her next heat is due:
| Bred on | Foal due | Normal window |
|---|---|---|
| Jan 1 | Dec 7 | Nov 17 – Jan 6 |
| Jan 15 | Dec 21 | Dec 1 – Jan 20 |
| Feb 1 | Jan 7 | Dec 18 – Feb 6 |
| Feb 15 | Jan 21 | Jan 1 – Feb 20 |
| Mar 1 | Feb 4 | Jan 15 – Mar 5 |
| Mar 15 | Feb 18 | Jan 29 – Mar 19 |
| Apr 1 | Mar 6 | Feb 15 – Apr 5 |
| Apr 15 | Mar 20 | Feb 29 – Apr 19 |
| May 1 | Apr 5 | Mar 16 – May 5 |
| May 15 | Apr 19 | Mar 30 – May 19 |
| Jun 1 | May 6 | Apr 16 – Jun 5 |
| Jun 15 | May 20 | Apr 30 – Jun 19 |
| Jul 1 | Jun 5 | May 16 – Jul 5 |
| Jul 15 | Jun 19 | May 30 – Jul 19 |
| Aug 1 | Jul 6 | Jun 16 – Aug 5 |
| Aug 15 | Jul 20 | Jun 30 – Aug 19 |
| Sep 1 | Aug 6 | Jul 17 – Sep 5 |
| Sep 15 | Aug 20 | Jul 31 – Sep 19 |
| Oct 1 | Sep 5 | Aug 16 – Oct 5 |
| Oct 15 | Sep 19 | Aug 30 – Oct 19 |
| Nov 1 | Oct 6 | Sep 16 – Nov 5 |
| Nov 15 | Oct 20 | Sep 30 – Nov 19 |
| Dec 1 | Nov 5 | Oct 16 – Dec 5 |
| Dec 15 | Nov 19 | Oct 30 – Dec 19 |
What makes a mare’s pregnancy longer or shorter?
- Season: mares that foal in late spring and summer tend to have shorter pregnancies than those foaling in January or February. Extra daylight in the last third of pregnancy shortened it by about 10 days in one study.
- Foal’s sex: colts are often carried about 2 to 7 days longer than fillies.
- The individual mare: many mares repeat a similar length each year, so her past records are your best guide.
When is a foal premature?
Foals born before about day 320 to 326 usually need vet care right away. A pregnancy that ends before day 310 is counted as an abortion. If a mare goes well past day 365, or shows any sign of illness, ask your vet to check her.
What are the signs a mare is about to foal?
| Sign | Usual timing before foaling |
|---|---|
| Udder starts to fill | 2 to 4 weeks (maiden mares may not show it) |
| Muscles soften around the tailhead | 1 week to 1 month |
| Teats fill with milk | 2 days to 1 week |
| Milk drips | 1 day to 1 week |
| Waxing (beads of colostrum on the teats) | 1 to 4 days |
| Milk calcium rises | 1 to 1.5 days |
| Vulva relaxes | 12 to 24 hours |
In the last days, milk calcium test strips are a better guide than any mare foaling calculator. When calcium is above 200 ppm, about 54% of mares foal within 24 hours and 97% within 72 hours. Below 200 ppm, fewer than 1% foal in the next 24 hours.
How long does foaling take?
Foaling moves fast compared with other animals. In the first stage, which lasts about one to two hours, she is restless and may sweat, look at her flanks or get up and down. It ends when her water breaks.
The foal is usually born within 15 to 20 minutes after that. The placenta normally follows within one to three hours.
When to call the vet
- The first stage lasts more than an hour or two with no progress.
- The foal is not born within 30 minutes of her water breaking.
- You see the soles of the foal’s hooves pointing up, or anything other than two front feet and a nose.
- The placenta has not passed within 3 hours.
- The foal has not stood within about 2 hours or nursed within about 3 hours.
- She leaks a lot of milk before foaling.
These checks follow the foaling guide from Oklahoma State University Extension. For comparison, a cow carries for about 283 days in our cattle gestation table, while a goat’s pregnancy lasts only about 150. Our gestation chart shows every animal side by side.
Frequently asked questions
How long is a horse pregnant in months?
About 11 months on average. Counting the full normal range, a mare can foal anywhere from about 10.5 to just over 12 months after breeding.
What is the longest a mare can be pregnant?
Pregnancies up to about 365 days are not uncommon. Beyond that, ask your vet to check the mare, because a very long pregnancy can point to a problem.
What is the 1-2-3 rule for mares?
A common rule of thumb after foaling: the foal should stand within about 1 hour, nurse within about 2 hours, and the mare should pass the placenta within about 3 hours. Some vets allow a little longer, such as up to 2 hours to stand, so ask yours what to watch for.
Do mares usually foal at night?
Many do, often when the barn is quiet. That is one reason foaling cameras and milk calcium tests are so popular with owners.
Can I make my mare foal earlier?
Do not try to speed things up on your own. Inducing labor carries real risks to the foal and is a decision only your vet should make.
When should I move my mare to the foaling area?
About two to three weeks before her expected date, so she can settle in. A foaling stall should be at least 14 by 14 feet with deep, clean straw.