How Long is a Horse Pregnant? A Guide to Equine Pregnancy.

Pregnancy is a stressful time for us horse owners. We obviously want the best for our mares and can turn into helicopter (horse) parents during the pregnancy.

But, there is no point faffing around your horse, stressing whether everything is okay if you have no idea about the duration of pregnancy in horses and what to expect.

How long do horses stay pregnant? Horses are pregnant for about 11 months, but there are several factors at play.

In this article, I’ll elaborate on the answer to the question “How long are horses pregnant for?” and also give you more information on what to expect during each stage of the pregnancy. Having this info at hand means you will be able to properly care for your mare and keep an eye out for any discomfort or concerning symptoms.

A Mare’s Cycle

Before getting into the duration of a horse’s pregnancy, you first need to understand the complexities of a mare’s cycle.

Mares are seasonally polyoestrous – this means their cycles are influenced by the number of daylight hours. Her cycle will typically be kickstarted when she produces less melatonin once daylight hours increase.

Mares go through oestrous cycles during summer, spring, and autumn. Each cycle is about 22 days long and divided into two parts – oestrus and dioestrus. Oestrus is when the mare is “in heat”, and it lasts from three to seven days. During dioestrus, the mare is not receptive to the advances of stallions.

Mares don’t have an oestrous cycle during winter as a way to prevent foals from being born in harsh, cold conditions.

How Long Is A Horse’s Gestation Period?

how long is a horse pregnant? A horse is pregnant for about 326 to 345 days (11 months). Some mares may foal earlier than this and some might go past their foaling date.

Wild horses get pregnant during summer, and then foals are born almost a year later – in late spring or early summer. This is a survival adaptation – foals have the best chance of surviving when temperatures are mild and food is abundant.

As I mentioned above, mares are seasonally polyoestrous, meaning they go into heat several times during the year, especially in the warmer seasons.

Horse breeders who want to have foals earlier in the year – outside of the mare’s natural breeding season – have used artificial lights to trick mares into releasing hormones that cause oestrus. This then leads to successful breeding earlier, and a foal born earlier the next year.

This practice is most commonly done by racehorse breeders. It can be a challenge earlier in the year due to bad weather of you do not have a healthy foal, this is why we aim for late spring here at Strathorn. When a foal is born earler in the year, it will look bigger than it’s competiton in the ring or on the track, which is why many competition stables aim for this.

Factors influencing gestation length

Female horses can be influenced by environmental conditions, which will change their gestation length.

Here are some factors that have an influence on the duration of the pregnancy:

• Temperatures: If you breed earlier in the year when it is still cool, the mare is likely to have a longer pregnancy so that the foal is born in optimal conditions. If you breed later in the year when it is warmer, the gestation period will be shorter.
• Environment: ensure a stres- free environment for your pregnant mare. Try not to introduce new horses, or change their routine too much.
• Weight: Thinner mares tend to have longer gestation; overweight mares will have a shorter pregnancy.
• Sex of the foal: This may be a bit of an old wive’s tale, but breeders are convinced that colts are carried for about a week longer than fillies.

Gestation Period Trimesters

There are three trimesters during a mare’s gestation period. Let’s look at each in greater detail.

First Trimester

Day 0 – 114

The first trimester begins at conception. A vet can confirm whether the mare is pregnant within two weeks after mating.

About 30 days later is when the heartbeat of the foal will be heard via ultrasound. At this stage, the vet will also be able to confirm whether the mare is pregnant with twins (twin foals are an incredibly rare occurrence).

After about three months, the vet will be able to determine whether the pregnancy is viable. The gender of the fetus can also be determined at this stage.

Second Trimester

Day 114 – 226

At around 114 days (four months) the foal begins to grow fast. During this stage, pregnant mares will require more nutrition to maintain their body condition. The mare will also receive equine herpesvirus (EHV-1) vaccinations and deworming during this trimester.

Third Trimester

Day 226 – 340

At the six-month mark, the third stage of pregnancy begins. Feeding and supplementation should increase during this trimester to prepare the mare for the foaling day. The vet may also recommend some additional vaccinations during this trimester, including flu, tetanus, and rabies.

During this time, as the mare nears giving birth, her exercise should be reduced and she should be kept as stress-free as possible.

Foaling

In the dayprior to foaling, a mare’s behavior often changes as her body prepares for birth. At Strathorn Farm, we’ve noticed our Clydesdale mares becoming more restless, frequently pawing the ground, circling, or lying down and getting back up.

They may separate themselves from other horses, seeking a quiet spot, and can appear more irritable or anxious. Physical signs also give us clues—udder enlargement, waxing of the teats (a sticky, colostrum-like discharge), and relaxation of the tailhead and pelvic ligaments.

Some mares will lose their appetite or spend more time looking at or nuzzling their sides. We always keep a close eye on these behaviors, as they’re clear indicators that foaling is imminent, allowing us to step in quickly if any assistance is required.

As mentioned, the average gestation period lasts around 11 months, and the female horse can go into labour from 326 to 354 days after conception.

Sometimes this is shorter or longer, but foals born before 300 days have little chance of surviving and require intensive care.

Once labour starts, the mare will grow increasingly restless. She may paw at the ground, walk around, and stand up and lie down several times.

Eventually, she will give birth laying down. The duration of the birth will depend on whether the mare is a first-time mother or has had many foals before. First-time moms have longer labour than experienced mothers.

The horse’s amniotic sac is a thin, transparent membrane that surrounds and protects the foal during pregnancy, acting as a cushion within the mare’s uterus. It’s filled with amniotic fluid, which supports the foal’s development by providing a sterile environment, maintaining temperature, and allowing for free movement as the foal grows.

During labor, the amniotic sac is often the first structure to emerge, appearing as a white or bluish bubble at the mare’s vulva. If the sac doesn’t break naturally, it may need to be carefully opened to ensure the foal can breathe—this is a rare occurrence but one we’re always prepared for here at Strathorn Farm.

Monitoring the sac’s condition is vital, as abnormalities in the fluid or membranes can indicate potential problems requiring veterinary attention.

Checking For Horse Pregnancy

Your mare and the stallion have done the deed – but how do you know if it was successful?

Horses will only begin to show signs of pregnancy after about three months. Luckily you don’t have to wait that long, as an ultrasound can be done after just two weeks to confirm conception. A vet can also feel for a small embryo after about six weeks.

Blood and urine tests can be used two months after the mating occurred to test for pregnancy.

It is crucial that you have regular vet appointments once your horse is pregnant. That way the vet can monitor the health of your mare and the unborn foal.

Optimal Diet for Pregnant Horses

Here at Strathorn Farm stables, we take feeding our pregnant Clydesdale mares seriously to ensure both the mare and foal stay healthy throughout the pregnancy. We start by providing a balanced diet rich in high-quality forage, which forms the foundation of their nutrition.

Good homegrown haylage or grass provides essential fiber and nutrients to keep their digestive systems working properly and prevent colic. We always make sure their haylage is free of mold and dust—Clydesdales are big animals, but their guts can be surprisingly sensitive.

In addition to forage, we supplement with a specially formulated concentrate designed for broodmares. These feeds are packed with vitamins and minerals like calcium, phosphorus, and magnesium, which are vital for strong bone development in the foal and maintaining the mare’s own bone health.

We pay particular attention to protein levels, as pregnant mares need more to support fetal growth. Our Clydesdales are large horses, so we’re careful not to overfeed grain to avoid excess weight gain, which could lead to complications during foaling.

We also add omega-3 fatty acids, often through flaxseed or linseed oil, to support fetal brain and joint development. For added insurance, we include a vitamin E supplement, especially during the later months, to boost immune function for both the mare and the unborn foal.

Fresh, clean water is a non-negotiable, and we check buckets and troughs daily to make sure the mares are drinking enough—hydration is key to preventing issues like impaction colic.

Finally, we keep a close eye on their body condition throughout pregnancy. With Clydesdales, it’s easy to overlook weight gain under all that feather and bulk, so regular checks help us stay ahead of any changes. We work closely with our vet to monitor their progress and adjust feeding as needed.

At Strathorn, we believe in proactive care, and that includes making sure our pregnant mares are getting exactly what they need to grow healthy, strong foals. Read all about what to feed horses here

FAQs

When can most mares start breeding?

A mare can start breeding when she reaches 18 months. However, this is considered too early by many reproductive equine practitioners. They state that the best time for breeding is when a mare is four years old (or older).

Mares can get pregnant even when they are nearing 20 years of age, so there is no rush for you to begin breeding with your horse.

How long does it take a mare to give birth?

Mares who are foaling for the first time can take about an hour of intense labour to give birth. Experienced mares are able to foal in about 18 minutes. If an experienced mare is in labour for 45 minutes or longer, it may be time to call in a vet.

What supplements should I give my pregnant mare?

Your mare will only really need extra nutrition during the last trimester. Prior to this you just need to ensure that she is maintaining her body condition. During the third trimester, you can begin to supplement with copper, zinc, vitamin A and vitamin D.

Does the gestation period vary between different horse breeds?

The gestation period in horses typically lasts around 11 months, or approximately 340 days, but it can vary slightly depending on the breed. Larger breeds, such as our Clydesdales here at Strathorn Farm, often have slightly longer pregnancies compared to smaller breeds like ponies.

Factors such as genetics, nutrition, and the individual mare’s health can also influence the length of gestation. For those interested in learning more about different horse breeds and their characteristics, including British breeds like the Clydesdale, you can find more information here.

What should I do if my mare goes beyond the average gestation period?

If your mare goes beyond the average gestation period of around 340 days, don’t panic—many mares, especially first-time mothers or larger breeds like Clydesdales, often carry their foals for a bit longer. It’s not uncommon for gestation to last up to 370 days, and in most cases, the foal is perfectly healthy. However, it’s always a good idea to keep a close eye on her during this time.

First, monitor her closely for signs of impending labor, such as udder development, waxing (colostrum buildup), or relaxation of the tailhead and pelvic ligaments. Make sure she has a clean, quiet foaling area ready. Continue feeding her a balanced diet and ensure she stays hydrated, but avoid overfeeding as excess weight could make foaling more difficult.

If she shows no signs of labor after 365 days or if you notice any unusual behavior—such as discomfort, lack of appetite, or restlessness without progressing to labor—contact your vet immediately. Your vet may want to perform an ultrasound to check the foal’s position, heartbeat, and overall health.

At Strathorn Farm, we always prioritize a proactive approach, so we recommend regular checkups with your vet throughout late pregnancy to catch any issues early. While most overdue mares foal without complications, it’s best to have professional guidance to keep both mare and foal safe.

How often should a pregnant mare be checked by a veterinarian during pregnancy?

A pregnant mare should be checked by a veterinarian at least three times during her pregnancy, but more frequent visits may be necessary depending on her age, health, and any prior complications. At Strathorn Farm, we schedule routine checks to stay ahead of any potential issues.

The first check typically happens early in pregnancy, around 14–16 days after breeding, to confirm pregnancy and detect twins—since mares can sometimes conceive twins, which can lead to complications. A follow-up scan around 25–30 days confirms the heartbeat and monitors fetal development.

The second major check occurs around 90 days to ensure the pregnancy is progressing normally. At this point, we often discuss nutrition adjustments and supplements to support the growing foal. If there are any concerns, additional ultrasounds may be recommended.

The final routine check is usually done in the last trimester, around 270 days, to assess the mare’s condition and prepare for foaling. During this visit, we also review her diet, vaccination schedule, and deworming plan to make sure everything is in place for a healthy delivery. Of course, if anything seems off at any stage, we don’t hesitate to call the vet for an extra look—it’s always better to be safe than sorry!