Horses are pregnant for about 340 days, roughly 11 months, but the normal range is far wider than the average suggests. Kentucky Equine Research states that anywhere between 320 and 370 days is completely within normal limits, and describes equine gestation as far more variable than that of other mammals. A mare at day 355 is not late. She is inside the range, and so is a mare at 330 and a mare at 365. The number that predicts your mare's foaling date best is not the species average. It is her own previous gestation lengths, if you have them.
Why the average misleads
Three hundred and forty days gets quoted as though it were a due date, and it is not. It is the middle of a distribution that is about fifty days wide.
This matters because of what owners do with the number. A first-time breeder marks day 340 on a calendar, watches it pass, and starts phoning people. Meanwhile the mare is doing something entirely normal, and the anxiety burns through exactly the attention and sleep that will be needed when something actually happens. Published gestation figures from the sources consulted here range from an average of 335 to 340 days with a range of 320 to 362, out to a stated normal band of 320 to 370. The variation between sources is itself the point: nobody can give you a date.
What the day count actually means
This is the table most gestation articles do not give you, and it is the one that answers the question behind the question. It comes from Kentucky Equine Research's work on prematurity and gestational length.
| Day of gestation | What it means | What happens |
|---|---|---|
| Before 300 days | Foals born this early to healthy mares rarely survive | Surfactant, which supports lung function, does not work in foals the way it does in human infants. This is why the outcome is poor rather than simply early. |
| 300 to 320 days | Premature | These foals frequently require neonatal intensive care, usually at a veterinary hospital. High-quality nursing care often sees them through. |
| At or before 320 days | Classed as premature | The formal threshold. |
| 320 to 342 days | Inside normal, improving odds | Viability improves substantially past the 320-day mark. |
| 342 days and beyond | Best odds | The closer to 342 days or more, the better the chance the foal lives. |
| Beyond 370 days | Generally not a problem in itself | Postmature foals are sometimes smaller than expected, associated with delayed uterine development rather than with being late. |
Two things follow from that table. First, early is dangerous in a way that late is not. Second, the panic point that owners actually experience, somewhere around day 350 to 360, sits in the safest part of the entire distribution.
What shifts gestation length, and roughly how much
The factors are well documented and mostly small individually:
- Fetal sex. Gestation for colts commonly runs a few days longer than for fillies, with published figures in the range of about two to seven days.
- Season and daylight. Mares bred later in the year, through spring and summer when days are longer, tend toward shorter gestations. Mares carrying through the short days of winter tend to run longer.
- Mare age and parity. Age and whether she has carried before both influence length.
- Breed. Breed averages differ, which is part of why published ranges differ between sources.
- Nutrition and management during pregnancy.
None of these lets you calculate a date. Stacked together they explain why the range is fifty days wide, which is a different and more useful thing to know.
Your mare's own history beats the breed average
Here is the practically useful part, and it is the reason record-keeping matters more in a breeding operation than any calculator.
Individual mares tend to be reasonably consistent from pregnancy to pregnancy. A mare who carried 355 days last time is a better bet to carry long again than the species average is to predict her at all. That means the single most valuable thing a breeder can own is a written record: date of the covering or insemination, confirmed ovulation date if your veterinarian scanned her, the actual foaling date, and the resulting day count, for every pregnancy.
If this is her first foal, you do not have that record and no one can build it for you. Say so out loud, plan for a window rather than a date, and start the record this year. By her second or third foal you will know more about her than any published average does.
An honest caveat on the calculators: a gestation calculator, including the widely used one published by The Horse, gives you a projected date from a breeding date. That projection is a midpoint, not a deadline, and it should be treated as the center of a month-wide window.
What "overdue" does and does not mean
"Overdue" is one of the least useful words in a breeding barn, because it implies a deadline that does not exist.
A mare past 340 days is not overdue. A mare past 370 days is unusual and worth discussing with your veterinarian, but the published position is that foals born after 370 days generally do not present a problem, and that some are actually smaller than expected rather than larger.
What is worth acting on is not the day count. It is a change. A mare who has been comfortable and is suddenly not, a mare running milk days before anything else changes, a mare showing signs you cannot place: those are veterinary calls regardless of what the calendar says. The calendar is background information. The mare is the data.
What to watch instead of the calendar
Stop counting and start observing. The reliable signals are physical and behavioral, not numerical: udder development and filling, relaxation of the muscles around the tail head, changes in vulvar conformation, waxing of the teats, changes in behavior and appetite, and restlessness. What the approaching signs look like, and which ones are false alarms is the piece that covers those in detail, and it is worth reading well before you think you need it, because mares who wax and then wait four days are common enough to be routine.
Two other things belong in place before the window opens. The foaling kit should be assembled and in the barn, not on a shopping list. And what a red bag delivery is and why it is a minutes-count emergency should be read now, while you are calm, because it is not something to look up while it is happening.
When to call your vet
None of the following is a treatment instruction. Each is a reason to call.
| What you see | Why it matters |
|---|---|
| Signs of foaling before 320 days | Premature delivery. Foals in the 300 to 320 day band typically need hospital-level neonatal care, and arrangements are better made in advance than at 3am. |
| Any sign of labor before 300 days | Outcomes for foals this early are poor and the mare needs veterinary attention regardless. |
| Milk running days before any other sign | Loss of colostrum before the foal is born is a genuine problem, because the antibodies in that first milk cannot be replaced by later milk. |
| Vaginal discharge, or a mare who seems systemically unwell at any stage | Needs assessment, not observation. |
| Colic-like behavior in a late-term mare | Pre-foaling restlessness and colic can look alike, and telling them apart is a veterinary job. Why you often cannot tell the difference covers what this actually looks like. |
| Past 370 days with no signs | Uncommon rather than dangerous, but worth a conversation and possibly an examination. |
| Labor that starts and does not progress | Equine deliveries are fast. A stalled delivery is an emergency. |
What this article cannot tell you
It cannot tell you when your mare will foal. Nobody can, and anyone offering you a date is offering you a midpoint dressed up as a prediction.
It also cannot tell you what a specific day count means for a specific mare, because that requires someone who knows her history, has examined her, and can see her. That is your veterinarian, and the right time to have the conversation is well before the window opens rather than during it.
If you are earlier in the process than this, the decisions that shape everything downstream are whether to breed at all and how to choose a stallion for your mare.
FAQ
How long are horses pregnant?
About 340 days on average, roughly 11 months. Kentucky Equine Research describes 320 to 370 days as completely within normal limits, and other sources cite an average of 335 to 340 with a range of 320 to 362.
My mare is at 355 days. Is she overdue?
No. Day 355 is comfortably inside the normal range and inside the band with the best outcomes. Foals born closer to 342 days or beyond have better odds than those born earlier, and foals born after 370 days generally do not present a problem.
At what point is a foal considered premature?
A foal born at or before 320 days of gestation is considered premature. Foals born between 300 and 320 days frequently require neonatal intensive care. Foals born before 300 days to healthy mares rarely survive.
Does the foal's sex change the length of the pregnancy?
Slightly. Gestations carrying colts commonly run a few days longer than those carrying fillies, with published figures around two to seven days. It is not enough to plan around.
Are horse gestation calculators accurate?
They give a reasonable midpoint from a known breeding date, and nothing more. Treat the result as the center of a window of roughly a month rather than as a due date, and watch the mare rather than the calendar.
Sources consulted: Kentucky Equine Research, "Gestation in Mares: What Is Premature? What Is Overdue?" and "Gestation Length Variation in Mares"; Merck Veterinary Manual, management of reproduction in horses; The Horse, mare gestation calculator; Mad Barn, "Gestation in Horses" and "Abnormal Gestational Length in Foals"; UC Davis Center for Equine Health. This site is written by a horse owner, not a veterinarian. Nothing here is a diagnosis or a treatment instruction.