Horse Gestation: How Long a Mare Is Pregnant

Author: Elliott Garber, DVM

A mare is pregnant for about 340 days, roughly 11 months, though a healthy foal can arrive anywhere in a normal window of about 320 to 370 days. There is no single “due date” that every mare hits, so it is smarter to plan for a foaling window than to circle one day on the calendar. Season, individual mare, and even the sex of the foal all shift the number by days to weeks.

How long is a mare actually pregnant?

The figure most veterinary and university sources agree on is an average around 335 to 340 days, with a broad normal range of roughly 320 to 370 days. Extension Horses and university extension foaling publications describe the same picture: mares vary a lot from one to the next, but most individual mares tend to repeat their own pattern year after year. If your mare carried her last foal to 355 days, she will probably run long again.

That variability is why experienced breeders talk about a foaling window rather than a due date. A foal born at 320 days is not “early” in any worrying sense, and one at 365 days is not automatically overdue. What matters far more than the exact day is whether the foal is developed and the mare is progressing normally as she approaches the end. Foals born before roughly 320 days are considered premature and need close veterinary attention, so track the breeding date so you know where you stand.

Because the range is so wide, count forward from the breeding or ovulation date your vet recorded, add about 340 days for a rough target, then start watching closely from about 320 days on. You can keep the breeding date, confirmation checks, and expected window on your mare’s Creatures profile so nothing lives only on a whiteboard in the barn.

What makes gestation longer or shorter

Several factors nudge the length, and knowing them helps you set expectations for a particular mare.

Season and daylight. This is the biggest predictable factor. Mares due to foal in late winter or early spring tend to carry their foals about 5 to 10 days longer than mares foaling later in the season, and gestation length tends to shorten as the daily light phase lengthens through the foaling season, according to the Merck Veterinary Manual. This is the same light sensitivity that makes mares seasonally cyclic in the first place. Breeders chasing early foals sometimes put mares under artificial lighting, which can shorten early-season gestations by around 10 days on average, but that is a management program to plan with your vet, not something to improvise.

The individual mare. Genetics and maternal line matter. Some mares are simply long carriers and some are short, and they tend to be consistent. Your best predictor for a given mare is often her own history.

Foal sex. Studies summarized by extension services show colts tend to gestate a few days longer than fillies, on the order of 2 to 7 days.

Nutrition and body condition. A mare’s plane of nutrition has been shown to have some influence on gestation length. This is one more reason to keep a broodmare in steady, moderate condition rather than letting her get thin or fat.

Confirming the pregnancy and the vet-check schedule

You do not have to wait months to know whether a mare is in foal. The standard first step is an ultrasound about 14 to 16 days after breeding. This confirms the pregnancy and, just as importantly, checks for twins, according to the Colorado State University Equine Reproduction Laboratory. Equine twins are dangerous: mares are poorly built to carry two, and twin pregnancies commonly end in abortion or serious foaling problems, so vets specifically look for a second embryo early, when there is still a safe window to manage it.

From there, a typical monitoring schedule (which your vet will tailor) includes a recheck around 25 to 30 days to confirm a heartbeat, another look near 45 to 60 days, and a check around 60 to 70 days when fetal sexing is possible if you want it. The exact rhythm is your veterinarian’s call based on the mare and her history, per the MSD/Merck Veterinary Manual. The point of the early schedule is to catch problems (twins, early loss) while there is still time to act.

Log each confirmation and scan as a health record on the mare’s profile. A clear record of the breeding date, the 14-day scan, and each recheck makes the whole pregnancy easier to manage and gives your vet the timeline at a glance. If you are not sure where these live, the profile tabs help article walks through where records and reminders sit.

Vaccinations during pregnancy

Vaccinating a pregnant mare does two jobs: it protects the pregnancy itself (mainly against abortion caused by equine herpesvirus) and it loads the mare’s colostrum with antibodies the newborn foal absorbs in its first hours of life. Both are on a schedule, and both should be set by your veterinarian. Do not use this article to pick products or timing on your own.

The one schedule specific to pregnancy is the equine herpesvirus (EHV-1, rhinopneumonitis) abortion series. A killed EHV-1 vaccine labeled for prevention of abortion is typically given at 5, 7, and 9 months of gestation, as described by the Merck Veterinary Manual. EHV-1 can cause mares to abort, sometimes in outbreaks, and this series is the main tool to reduce that risk.

The second piece is the pre-foaling booster. Core and risk-based vaccines that the mare gets annually are boosted about 4 to 6 weeks before her due date so that antibody levels in her colostrum are high when the foal nurses, again per Merck. Which vaccines are in that pre-foaling set depends on your region and the mare’s risk, which is exactly why it is a vet conversation and not a fixed list. For the broader picture of core versus risk-based vaccines across a horse’s life, our horse vaccination guide covers the framework, but the pregnant-mare schedule specifically belongs to your veterinarian.

Record every dose, the product, and the gestation month it was given. The reminders feature is genuinely useful here, because missing the 7-month or 9-month EHV booster is an easy mistake in a busy barn.

Feeding the mare through late gestation

Here is a fact that surprises new broodmare owners: for most of the pregnancy, a mare’s nutritional needs barely change from maintenance. Roughly 75 percent of fetal growth happens in the last trimester, so that is when her energy, protein, and mineral requirements climb, according to University of Nebraska extension and other extension sources. Overfeeding a mare in early and mid pregnancy just makes her fat, which is its own problem.

In the last three to four months, energy and protein requirements rise meaningfully above maintenance, and mineral requirements increase as well. Complicating things, the growing foal takes up room in the abdomen, so the mare may physically eat less forage right when she needs more nutrition. That combination is why late-gestation broodmares are often moved onto a properly balanced broodmare feed alongside good forage, rather than just given more hay.

Do not treat any of this as a formula. Body condition, forage quality, the mare’s age, and her workload all change the math. The safe approach is to keep her in moderate body condition and let your vet or an equine nutritionist set the ration. Our horse feeding guide and hay guide cover the general feeding framework that late-gestation nutrition builds on.

Preparing for foaling and the signs to watch

As the foaling window opens (from about 320 days on), shift into preparation mode. Get the mare settled into a clean, safe foaling area she is comfortable in, ideally a few weeks ahead so she is not stressed by a last-minute move, and make sure you have your vet’s number and a plan for how you will monitor her.

Watch the mare, not just the calendar. Classic approaching signs include udder development and “bagging up” in the last weeks, then waxing (small beads of colostrum at the teat ends) usually in the last day or two, relaxation of the muscles around the tailhead, and often a drop in behavior and appetite as labor nears. None of these is a precise clock, which is why so many breeders lose sleep during foaling season.

The single most important thing to understand about equine foaling: the actual delivery is fast and, when it goes wrong, it goes wrong quickly. If active labor is not progressing normally, or anything looks off, that is an emergency and you call your veterinarian immediately rather than waiting. Do not attempt to manage a difficult delivery from an internet article. For the full walkthrough of stages of labor, what normal looks like, and exactly when to intervene, read our companion foaling guide before your mare is due.

If you are building out a broodmare’s records for the first time, adding the animal and then logging her breeding date and each vet check gives you a clean timeline for the whole pregnancy. Naming a foal on the way? The horse name generator is there when you need it.

Frequently asked questions

How long are horses pregnant?

About 340 days on average, roughly 11 months, with a normal range of about 320 to 370 days. There is no exact due date, so plan for a foaling window and start watching closely from around 320 days.

Can a mare foal early and still be fine?

Foals born within the normal range (down to about 320 days) are generally considered full term. Foals born before roughly 320 days are premature and need veterinary attention. A foal arriving a couple of weeks “early” relative to your rough count is often just the mare’s natural pattern or a seasonal effect, not a problem.

Why is my mare going past her due date?

A “due date” is only an estimate off a 340-day average. Many mares, especially those foaling in late winter or early spring, naturally carry 5 to 10 days longer. As long as your vet is happy with how she looks, a longer gestation within the normal range is usually not a concern. Flag anything unusual to your veterinarian.

When should I first confirm the pregnancy?

Around 14 to 16 days after breeding, by ultrasound. This early check confirms the pregnancy and, critically, screens for twins while they can still be managed safely.

Do I need to change how I feed her right away?

Usually not. A mare’s requirements stay close to maintenance until the last trimester, when most fetal growth occurs. Keep her in moderate condition, then step up energy, protein, and minerals in the last three to four months under your vet’s or a nutritionist’s guidance.

Do this next on Creatures

Whether you are dialing in day-to-day care, planning a breeding, or shopping for your next horse, Creatures is the records, marketplace, and directory layer to do it in one place.