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.
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?
A horse is pregnant for about 326 to 345 days (11 months). Some mares may foal earlier than this.
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.
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.
- 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.
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.
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.
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.
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.
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.
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.