Whether you are a breeder or just raise horses, it’s important that you are prepared for the horse pregnancy.
When horses are bred, the female horse (called a mare) produces a baby horse (called a foal). This is because they have either been naturally or artificially inseminated by a male stud (called a stallion) during the mare’s estrus, also known as in “heat.”
A mare can have a foal once a year when they are bred and can even become pregnant as young as 18 months old. However, the ideal age when breeding a mare should be at least four years or older.
Also, keep in mind that mares can be bred and carry a foal all the way into their late 20’s, so there is no reason to breed too early, especially when it can have potentially detrimental effects. Be patient and wait for your mare to fully mature before attempting to breed her.
What is the Normal Length of a Pregnancy (Gestation Period)?
Horses may be pregnant for as long as 326 days to 354 days. There are even some cases where gestation for a mare has reached as high as 365 to 370 days.
Don’t worry though, even if your pregnant horse is carrying for an exceptionally long time, as long as it’s close to a year in length, it’s considered a safe amount of time. By the same token, if your mare is healthy and the pregnancy of the horse has been uneventful all throughout the process, yet she delivers earlier than 11 months, that can be normal too.
It won’t usually be labeled “premature” unless there were difficulties or something unusual about the pregnancy and gestation period. Foals that are born prematurely will need a vet on hand to make sure everything goes smoothly and the foal is safe. If your mare foals before the gestation period has reached at least 300 to 310 days, the vet will consider the pregnancy or gestation period “aborted,” as most likely, the foal will be too young and undeveloped to survive.
Additionally, seasonal effects can impact the length of your mare’s gestation period. Mares that are bred earlier in the year (usually the first three months of the year or so) will often carry their foal a week 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.
Again, this is considered a normal part of gestation and is not a cause for worry. Some breeders even encourage a shorter gestation period by exposing the mare to longer artificial days during the final trimester. This may reduce the mare’s gestation period by as much as 10 days.
Other things that can impact a mare’s gestation period are factors like whether the foal is a colt (male foal) or a filly (female foal). The gestation period for colts can run anywhere from 2 to 7 days more in length than gestations for fillies. Body weight can also contribute to gestation periods, with mares that are thinner carrying their foals longer than mares with at a healthier weight when bred.
For the most part, a pregnancy (or gestation period) in a mare will fall somewhere between 335 and 345 days. This period works out to roughly 11 months of gestation. That 11 months can be broken up into three trimesters which is similar in length to most pregnancies in mammals.
What to Do If You Think Your Mare is Pregnant
Just like human females, mares experience cycles of heat. If you notice they have not experienced their heat cycle like normal, it’s possible it is because the mare may be pregnant. Typically, a mare that has not conceived will go back into heat 17 to 20 days after the breeding attempt.
Mares may not show any other signs other than no estrus for at least three months, so the fact that she’s not in heat may be your only clue until you have your vet inspect her. If you suspect your mare is pregnant for any reason, or you have bred your mare with a stud (either naturally or artificially) and believe her to be carrying a foal, the first thing you should do is confirm the pregnancy.
You can have your vet confirm your mare is pregnant by using an ultrasound around two weeks after breeding. Blood and urine tests can also confirm pregnancy, but nothing may show up until 10 days past ovulation, if not longer.
Sometimes a vet may be able to tell a mare is pregnant by changes in her uterus that they can feel by manual assessment. These changes can be detected around 6 weeks into the pregnancy. Ultrasounds are currently the fastest and most efficient way to tell if your mare is pregnant, detecting pregnancies as early as 5 days into the gestation period.
Ultrasounds can also tell you the exact stage of gestation your mare is in, as long as she is not past day 45 in her pregnancy. You should also keep detailed records if you plan to breed your mare on a regular basis, because though every mare is different, they do tend to repeat behaviors during the foaling process from mare to mare. Keeping detailed records will help you catch the signs of pregnancy early and know when you can expect her to deliver.
Pregnancy Gestation Stages for a Mare
A mare will go through approximately three trimesters during her gestation period. The first trimester begins with ovulation and is confirmed about two weeks later with an ultrasound. Your vet will likely check for the presence of twins, as a mare that is carrying twins often will abort within the first 6 weeks of gestation. If twins are found, your vet may remove the second embryo so that the other embryo has the best chance at surviving. If no evidence of twins is found at the two-week mark, your vet may require a second ultrasound to check at the 30-day mark.
Very rarely is a mare able to carry two foals to term and when she does, there are often complications during the foaling process as well as greater risk of complications in the first two weeks with the foals. Also, the foals born will be smaller and of lesser weight (equal to the size and weight of one foal between them) which stacks the odds against them growing and progressing in a healthy manner. Most of the time, twins occur because the mare has ovulated twice, one egg from each ovary.
Very rarely does an embryo split to produce identical twins. Regardless of how it happened, a mare carrying twins is not ideal and may endanger the mare if you try to allow her to carry them to term. Once your mare reaches the 90-day mark of gestation, your vet can check for the sex of the foal and give your mare her first dose of dewormer.
The second trimester starts roughly around day 114 and continues until the third trimester, which begins around day 226. During the second and third trimesters, your vet will give additional doses of deworming and check for the sex of the foal again if it’s not already known. Vaccinations are also given during these two trimesters, and in the third trimester, you will need to feed your mare more and increase her nutrition, so her body is prepared for the developing foal.
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 unpredictable, or you don’t have any prior history on the mare to begin with. The kits help breeders to estimate the time of the foal arriving within 12 to 24 hours, although it’s more a case of estimating when the foal will not arrive.
Basically, if the test results are negative to changes in the mare’s magnesium and calcium levels, breeders can rest a little easier knowing the foal will most likely not arrive for at least another 24 hours. This is helpful for breeders to know when they should be closely monitoring the mare.
Tips for Helping Your Mare Throughout Gestation
Caring for your mare is critical during pregnancy, especially the first 30 to 60 days. They should be in good health before you allow them to get pregnant and they should be at a healthy weight for their size and frame at the start. A mare that is too thin will struggle to get pregnant as well as carry to term.
Remember that stress is a killer, including a killer of pregnancy. Your mare can spontaneously abort if they are anxious or if they fall ill and cause stress to their body and health. Keep your mare close to home and only transport if necessary, and make sure she is kept safe from other horses. Other horses can pose a risk and cause unexpected injury or illness, especially if they are not horses your mare is usually around on a day-to-day basis.
You shouldn’t have to change any behaviors with your mare until around the 7-month mark in gestation. Up until that point, it is safe to engage her in moderate exercise and to ride her as you normally would and feed her a normal diet. Once she is into her later term of gestation, then you will need to alter your riding and exercise habits and increase her food. You also shouldn’t have to