The length of a pregnancy varies greatly across the animal kingdom. Our canine friends have about a 2-month gestation, a human pregnancy lasts about 40 weeks, and African elephants are pregnant for about 22 months. But what about horses? How long are they pregnant?
The average gestation length for a mare can last 320 to 380 days, with the average being 338 to 345 days.This means that a horse is pregnant from 11 to 12 months. Aside from knowing how long a horse’s pregnancy will last, there is much more to be learned for those considering breeding their horse and making sure she has a healthy, safe pregnancy.
The Cycle of a Mare
As a horse owner, it is important to know a mare’s cycle and what to expect. This is especially true when you plan to breed her. Mares are seasonally polyestrous, so understanding the seasons is an important part of the process.
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The Summer Solstice – The summer solstice is the longest day of the year and the peak of a horse’s natural breeding season.
The Autumn Equinox – During the autumn equinox, there is equal light and dark during the day. During this time, mares are transitioning from summer ovarian cycles and halting for the upcoming winter.
The Winter Solstice – The winter solstice is the shortest day of the year. During this time, mares will be in deepest anestrus, a time of reproductive incompetence.
The Spring Equinox – Like the fall equinox, the spring equinox also brings a time of equal light to dark during the day. But during spring the mare will be in a springtime transition back to normal ovarian cycles.
Factors That Can Affect Gestation
A typical pregnancy in horses can last for 320 to 380 days, but generally lasts around 11 months. There are several factors that can affect how many days a mare is actually pregnant.
Season: A mare’s gestation period can be impacted by the season. Mares that have been bred during the first quarter of the year will often carry their foals for slightly longer periods than expected, while mares bred during seasons of longer days may have slightly shorter gestational periods.
Gender: The gender of the foal can also affect gestation. Colts tend to gestate 2 to 7 days longer than fillies.
Body Weight: A mare’s body weight can affect how long the pregnancy lasts. Mares that are thinner tend to carry their foals longer than those that are heavier.
Health: As a general rule, only healthy mares should be considered for breeding. Those with underlying health issues are more likely to suffer from pregnancy complications and shorter gestation.
Maternal Age: Studies have shown that advanced maternal age has been related to lower fertility, higher rates of pregnancy loss, and mortality and morbidity in neonatal foals.
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The Stages of Pregnancy
Like humans, pregnant mares will go through three trimesters of pregnancy, with the first beginning at conception and being confirmed at around the 2-week mark. Horses give birth to one foal, but on very rare occasions, a mare may become pregnant with twins, which is highly undesirable in horses and often results in a poor outcome.
We break down the stages of a mare’s pregnancy and veterinary needs in detail below:
First Trimester
Day 0: Date of ovulation and beginning of the first trimester.
Days 14–16: Initial ultrasound to confirm pregnancy.
Days 25–30: Fetal heartbeat should be detectable to confirm foal viability and to evaluate a possible twin pregnancy.
Days 45–60: Pregnancy examination with ultrasound to reconfirm fetal heartbeat and viability of the pregnancy.
Days 60–90: First round of deworming should be given to the mare; possible fetal sexing via ultrasound.
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Second Trimester
Day 114: The second trimester begins.
Days 120–150: Fetal sexing can be performed via transabdominal ultrasound.
Day 150: First EHV-1 vaccination and second deworming for the mare.
Day 210: Second vaccination for EHV-1.
Third Trimester
Day 226: Beginning of the third trimester.
Day 270: Third vaccination should be scheduled for EHV-1.
Day 300: Nutritional increases for the mare to prepare for foaling, third deworming should take place, Caslicks should be opened, additional vaccinations should be administered per the recommendation of the veterinarian, and the mare should be moved to a foaling area.
Days 320–365: Normal foaling range.
Day 340: Average foaling date.
Tips for Keeping Your Mare Healthy and Safe Throughout Her Pregnancy
If you’re planning on breeding your horse or you have a mare that is already pregnant, it’s important to know how to keep them healthy during the pregnancy. Here are some tips.
1. Make Sure She Has a Clean Bill of Health Before Breeding
Mares should be completely healthy and at an appropriate weight before breeding is ever considered. Both underweight and overweight mares will be at risk of different complications during pregnancy, so she should be an ideal weight for her size when she’s bred. Avoid breeding mares of advanced maternal age, and always get a clean bill of health from your veterinarian before you allow her to become pregnant.
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2. Have Your Veterinarian Confirm the Pregnancy
It’s a good idea to have the pregnancy confirmed by your veterinarian. Not only can you lay your suspicions to rest and confirm a viable foal, but this will also allow you to rule out twins (though rare), which often require the elimination of one fetus due to unfortunate health outcomes for both mother and foals if the pregnancy is allowed to progress.
3. Keep Up With Veterinary Care Throughout the Pregnancy
Veterinary care throughout pregnancy is incredibly important. You will need to make sure the pregnancy remains viable and healthy and your mare will need regular deworming and vaccinations to make sure she and the foal remain as healthy as possible. If any issues do arise, your veterinarian will be able to direct you to the next steps and provide the best chance for both mother and baby.
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4. Only Transport Her If Necessary
Transporting can be stressful on any horse, so it’s best to avoid transporting a pregnant mare unless it’s absolutely necessary. Transporting also puts them at risk of injury if there were to be any accidents or issues loading and unloading.
5. Use Caution Around Other Horses
It’s important to keep your mare from stressing out too much during her pregnancy. You know your herd well, so be mindful and cautious when allowing her around other horses. Not only is it important to keep from stressing her out, but you want to prevent injury and illness as well, which may mean keeping her separated from certain horses.
6. Feed Her a Healthy, Balanced Diet
Your mare should be fed a well-balanced, nutritious diet but be careful not to overfeed. Supplementing with additional vitamins and minerals is unnecessary in mares that are fed a balanced diet. Any questions about your mare’s diet during pregnancy should be directed to your veterinarian.
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7. Prepare for Foaling and Newborn Care
As an owner, you should educate yourself on all the signs, both subtle and obvious, that your mare is getting ready to give birth, how to handle foaling, and how to properly care for the mother and the newborn foal after birth.
Signs of impending birth:
Filling of the udder (2 to 4 weeks pre-foaling)
Distension of the teats (4 to 6 days pre-foaling)
Waxing of the teats (1 to 4 days pre-foaling)
Dripping milk
Softening and flattening of the muscles in the croup
Relaxation of the vulva
Obvious changes in the position of the foal
Your 11-month waiting game will be over before you know it. To prepare, brush up on your foaling knowledge with the companion AAEP educational brochure, The Foaling Mare and Newborn. Your veterinarian will be happy to supply it and will also be able to answer any further questions you may have about caring for your expectant mare.
Conclusion
A horse’s pregnancy can last 11 to 12 months. More specifically, it ranges anywhere from 320 to 380 days, with 338 to 345 being the average. Pregnant mares must be healthy and receive regular veterinary care throughout the pregnancy to ensure a successful birthing process and a healthy, happy new foal. Horse owners should be well educated and fully equipped to handle their horse’s pregnancy before breeding is ever considered.
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