How Long Is a Horse Pregnant?

How long is a horse pregnant? Well, the short answer is 10 to 12 months, or from approximately 326 days to 354 days (although there have been cases where gestation for a mare has gone as long as 365 to 370 days). Most mares only carry one foal per pregnancy, although twins do occur on rare occasions. There is, however, quite a bit more to know if you’re considering breeding your horse.

Mares are seasonally polyestrous. To put this in lay terms, it means that the mare is something like a cat in that she will experience several cycles during a particular season. Like cats, mare cycle during periods of long daylight length. This is thought to be an evolutionary development to ensure that the mare will give birth at the most hospitable time, this being in Spring. Given these factors, a mare can only have one pregnancy a year and will usually only have one foal in a given year.

The Mare’s Cycle is Key

Understanding the mare’s cycle is integral to managing mares in general, and is absolutely crucial in planning for successful breeding. Since mares are seasonally polyestrous, the mare will be light responsive. This means that increasing daylight will cause her cycles to start by decreasing melatonin.

For horse breeders, important days to remember are:

• Summer Solstice - June 21, the longest day of the year and the peak of the natural breeding season
• Fall Equinox - September 21, when there is equal light and dark and the mares are turning off in Fall transition
• Winter Solstice - December 21, which is the shortest day of the year and mares are in deepest anestrus
• Spring Equinox - March 21 when there is equal light and dark and the mares are in Spring Transition¹

These dates are, of course, approximate. Temperature can also influence the onset of cyclicity as this is likely to be regulated in part by a neurotransmitter also involved in prolactin secretion. It is also thought that reduction of the opioid inhibition of the gonadal axis may also play a role in triggering the onset of the breeding season. Normal equine cycles typically occur around the Summer Solstice, which is the natural breeding season.

Seasonal effects can also impact the length of the mare’s gestation period. Mares that are bred earlier in the year (usually during the first quarter) will often carry their foal slightly longer than expected. Mares that are bred later in the year (during Spring and Summer when the days are longer) may have a gestation period that is shorter.² Other factors that may impact a mare’s gestation period are factors like whether the foal is a colt or a filly. The gestation period for colts can run anywhere from two to seven days more in length than gestations for fillies. Body weight can also contribute to gestation periods; mares that are thinner tend to carry their foals longer than mares with more weight.

Some breeders of performance horses occasionally manipulate a mare’s breeding cycle, using artificial light to stimulate the longer days of Spring and Summer. This causes the mare to go into heat earlier, which allows the foal to be born earlier in the year, often an advantage for the owners and managers of performance breeds.

Gestation Stages

Mares go through three trimesters during their gestation. The first trimester begins with conception and is generally confirmed at two weeks.³ During the first trimester, it is extremely important to have the veterinarian examine the mare to ensure her health and that of her foal.

At around 25 days, the veterinarian can perform an ultrasound that can detect the foal’s heartbeat and confirm vitality. It is also at this time that twins can be confirmed, if it is one of those extremely rare occurrences. If twins are found, the veterinarian may ask if the owner or manager desires removal of the second embryo in order to give the remaining one a better chance for survival. Mares sometimes abort twins within the first six weeks of gestation, which would obviously render the pregnancy unviable, with both foals being lost. At three months, the foal begins to look like a horse upon ultrasound examination; key features can be detected, and the gender of the foal can be determined.³

The second trimester begins at around day 114.³ During this time, the mare can begin to receive a dewormer and vaccinations. The mare’s feed should be increased to provide the needed nutrition to the fast-growing foal. By six months, the mare will begin to show.

At Day 226, the mare is in her third trimester. At this time, the vet visits should be stepped up again. Regular exercise can continue up until the seventh month. As the mare nears giving birth, it is important to keep her in a comfortable and stress-free environment, avoiding any major changes which might cause the mare to be anxious.

Leading Up to Foaling

Foaling day should arrive somewhere between day 326 to day 354, on average. There are test kits some breeders use to help anticipate foaling day, which can be useful especially if it’s the mare’s first foal, the mare’s foaling process is unknown.² In the days prior to delivery, the mare is likely to show signs that her body is getting ready to foal. Her udder is likely to look full, and may drip some milk. Her belly will appear lower than in weeks prior as the foal prepares to emerge.

The mare should be provided with a large stall with a lot of straw, fresh water and hay for her comfort. As the mare begins to go into labor, she will likely paw the ground and appear restless. She may get up and down a few times, but she will give birth laying down.^2,3 The amniotic sac will probably be the first part visible, followed by the head and legs. Once the amniotic sac is visible, it is generally a few minutes until the horse is born.³

Labor and Delivery

Most mares (greater than 85%) foal at night, which is probably a survival adaptation that allows the foal to be ready to run with the mare once daylight arrives. During the first stage of labor, the mare will be anxious. She may kick at her belly and will adopt nesting behavior. Many mares sweat during foaling, which is often referred to as the mare “heating up.” Wrap tail and clean perineal area. This stage usually lasts approximately one hour.

The second stage of labor usually lasts from 15 to 25 minutes. Continuous progress should reveal the foal’s front hooves, nose, ears, etc.² Although the prudent horse owner or breeder will have a veterinarian in attendance, the literature strongly suggests that it should be determined that the foal is breathing. This may be stimulated by using a blunt object to lightly massage the foal’s nostrils. If appropriate, the foal may be rubbed vigorously with a towel.³

Other tips and warnings consist of disinfecting any biologics with iodine.² It is advised when the foal is born, that the umbilical cord should not be cut immediately, as is the practice in humans. Some researchers believe that a certain amount of blood flows into the foal after birth through the umbilical artery.

In the third stage of labor, the foal has been born. The literature indicates that if the placenta is not passed within three hours it should be considered an emergency requiring a veterinarian’s attention. Within one hour, the foal should be standing, and should demonstrate the ability to nurse within two hours. The mare herself should usually require no post-partum care.

Emergency Situations

One of the more common foaling emergencies is the “red bag” appearance of the amniotic sac during the second stage of labor. In a normal foaling, the first thing to present is the amnion (or amniotic sac), a whitish membrane surrounding the foal. On rare occasions in which the placenta has detached from the uterine wall prematurely, there will be blood within the amnion, giving it a deep red appearance. This is a serious emergency that could result in the death of the foal.⁴ Other potential problems can arise with “breech deliveries” (versus the normal caudal presentation), but the seasoned equine veterinarian should be able to handle any of the above or other unforeseen circumstances with relative ease.