NewsHow 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 polyestrous periodically. To put it another way, the mare is similar to a cat in that she will go through numerous cycles throughout a given season. Mares, like cats, cycle during lengthy periods of daylight. This is assumed to be an evolutionary adaptation to guarantee that the mare gives birth at the most favorable time of year, which is in the spring. Given these conditions, a mare can only have one pregnancy per year and normally has one foal per 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.
Important dates for horse breeders to note are:
- Summer Solstice – June 21, the year’s longest day and the apex of the natural mating season
- Fall Equinox – September 21, when there is equal light and dark and the mares begin to switch off for the season
- Winter Solstice – December 21, the shortest day of the year, when mares are in full anestrus.
- Spring Equinox – March 21, when the light and dark are equal and the mares are in Spring Transition1
These are, of course, approximate dates. Temperature may also have an effect on the commencement of cyclicity, which is thought to be controlled in part by a neurotransmitter implicated in prolactin release. It is also hypothesized that decreasing opioid inhibition of the gonadal axis may play a role in starting the breeding season. Normal equine cycles take place around the Summer Solstice, which coincides with the natural mating 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
During their gestation, mares go through three trimesters. Conception occurs during the first trimester and is usually verified at two weeks.3 It is critical to have the mare examined by a veterinarian throughout the first trimester to safeguard her and her foal’s health.
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.
The mare is in her third trimester on Day 226. Vet visits should be increased again at this time. Regular physical activity may be continued until the seventh month. As the mare approaches her due date, it is critical to maintain her in a comfortable and stress-free environment, avoiding any dramatic changes that may cause the mare to become agitated.
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.
For the mare’s comfort, a roomy stall with plenty of straw, fresh water, and hay should be supplied. When the mare goes into labor, she will most likely paw the ground and look restless. She may move about a little, but she will give birth lying down.2,3 The amniotic sac will most likely be seen initially, followed by the head and legs. Once the amniotic sac is visible, the horse is usually delivered within a few minutes.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 suggestions and cautions include cleaning any biologics with iodine.2 It is recommended that the umbilical cord not be severed soon after the foal is delivered, as is the norm with humans. Some studies think that after delivery, a certain quantity of blood goes into the foal 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.
About NexGen Pharmaceuticals
NexGen Pharmaceuticals is a veterinary compounding pharmacy that provides sterile and non-sterile compounding services countrywide. NexGen, unlike other veterinary compounding pharmacies, concentrates on pharmaceuticals that are difficult to locate, are no longer accessible due to manufacturer discontinuation, or have yet to be commercially supplied for veterinary purposes, but nevertheless meet an essential need for our clients. We also specialize in wildlife medications, such as sedatives and their antagonists, and have numerous unique solutions to meet the immobilization and anesthetic needs of zoo animals and wildlife.
Our pharmacists are also encouraged to form excellent working connections with our veterinarians in order to provide better veterinary care. Such ties promote an ever-expanding knowledge base from which pharmacists and veterinarians may draw, allowing both to be much more successful in their respective jobs.
Disclaimer
This blog post contains general information and is meant to be used as a reference tool. It does not include all conceivable uses, actions, precautions, side effects, or interactions of the drugs depicted, nor is it meant as medical advice or diagnosis for individual health conditions, or for weighing the risks and advantages of taking a certain prescription. Any health concerns should be diagnosed and treated by your veterinarian. The Food and Drug Administration (“FDA”) has not examined the information and assertions, nor has the FDA authorized the drugs to diagnose, treat, or prevent illness. NexGen Pharmaceuticals prepares medications under the supervision of a veterinarian. NexGen Pharmaceuticals compounded veterinary medications are not designed for use in humans or livestock.
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