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What To Expect When Your Mare Is Expecting…

August 23, 2013

By Dr. Robyn Wilborn, 334-844-4490

Auburn, Alabama —

For horse owners who are relatively new to reproduction, many questions may arise during a mare’s pregnancy.  Hopefully this will serve to calm some of your fears and also alert you to potential signs of trouble should they arise.

Early Gestation

The total gestation period for horses can range from 330 to 345 days with some mares carrying their foal for almost a year.  The initial days after breeding are crucial for the developing embryo and will require hands on attention from both yourself and a veterinarian.  Early veterinary examinations include important milestones such as:

  • Palpation/ultrasonography at days 12-14 after ovulation.  The embryonic vesicle can be observed by ultrasonography to confirm pregnancy and also detect the presence of twins.  The early detection of twins is favorable to allow for successful elimination of one of the embryos and avoid risks associated with twinning.  
  • Palpation/ultrasonography at days 25-28 of gestation.  A heartbeat is visible at this time, confirming that development is on-track and twins can be definitively confirmed by this time. 
  • Palpation/ultrasonography to reconfirm the pregnancy at 45-90 days gestation.  Unfortunately, some mares lose their pregnancy during this early stage with loss rates ranging from 5-24%.  It is important to confirm that the pregnancy is viable and that everything is progressing normally as the mare transitions into mid-gestation.


Besides careful observation during pregnancy, not much changes for your mare during mid-gestation.  In fact, light to moderate riding is encouraged as a good source of exercise.  Feeding should be consistent with her normal caloric intake before pregnancy and in proportion to the amount of exercise she is getting.  The best way to determine her caloric needs is to evaluate her body condition periodically and adjust feeding amounts accordingly.  (Purina link)  For most mares, mid-gestation occurs throughout the summer and early fall months when grazing is still plentiful so additional calories are usually not necessary.

Limit exposure to other horses as much as possible during pregnancy to decrease her chances of contracting any disease that might jeopardize her well-being or potentially cause her to lose the pregnancy.  Many of these diseases are spread easily via the respiratory route, so young horses or those traveling to shows pose a significant risk for transmission of disease to the pregnant mare.  Hoof and dental care should continue along with routine deworming as recommended by your veterinarian.  Take care to only use dewormers that are labeled as safe for pregnant mares.

A mid-gestation visit from your veterinarian will likely include:

  • Palpation/ultrasonography to ensure that the mare is still pregnant and that the pregnancy is progressing normally.
  • Vaccination for equine herpesvirus (EHV-1) which should be repeated at 5, 7, and 9 months of pregnancy.  This vaccine is frequently referred to as “rhino,” which is short for rhinopneumonitis.
  • In the southeast where mosquitoes are prevalent well into the fall, a booster vaccine is recommended twice yearly (spring and fall) for the mosquito-borne diseases such as eastern/western encephalomyelitis and West Nile virus.

Late Gestation

The last four months of pregnancy will require an increase in calories to accommodate the rapid growth of the foal.  This usually corresponds to the late fall and winter months when grazing is no longer plentiful, emphasizing the need for additional calories.  Nutritional supplements are not required.  Simply adding concentrated feed (i.e. grain) alongside good quality hay or winter grazing will suffice.  Riding should be reduced during late gestation.  Mares housed on pasture will get plenty of light exercise on their own.  Transport stress and exposure to other horses should be limited as much as possible during late gestation.

A final visit from your veterinarian is recommended 4-6 weeks prior to foaling for several reasons:

  • Palpation to ensure that she is still pregnant and that the fetus is viable (active)
  • Removal of the Caslick’s (i.e. surgical opening of the vulvar lips if a Caslick’s suture was placed at the time of breeding).  This will allow the foal enough room for delivery once the big day arrives.
  • Booster vaccinations (aka “spring shots”).  In this part of the country we recommend vaccination against eastern/western equine encephalomyelitis, tetanus and West Nile virus +/- influenza.  Rabies vaccine is also recommended in this area.  It is important that vaccines are boostered 4-6 weeks prior to foaling not only for the mare’s health but more importantly for the health of the foal (ensuring that antibody levels in the mare’s colostrum provide optimal immunity for the foal when it arrives).

Potential Signs of Trouble

Hopefully you and your mare will experience smooth sailing through her pregnancy.  However, these signs can indicate a problem and your veterinarian should be called immediately if you notice any of the following:

  • Sudden or significant udder development prior to 10 months gestation
  • Vaginal discharge at any time during pregnancy
  • Colic signs, inappetance or weight loss

Expectant mare management is one of the most important factors to ensure the birth of a healthy foal and the future reproductive success of your broodmare.  Our goal is to work together with your regular veterinarian to provide the best possible care for your mare and to help answer any questions you may have along the way.

“What to expect when your mare is expecting…(Part 2); Steps to successful foaling” coming in February 2014.