Pregnancy Failure in Horses: Why Things Go Wrong

Pregnancy failure in mares is a very real risk; here’s how to prevent or handle it.

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Pregnancy Failure in Horses: Why Things Go Wrong
Even when a mare does get pregnant, there is no assurance she will carry the foal to term. | Photo:

Pregnancy failure in mares is a very real risk; here’s how to prevent or handle it

You’ve picked out a stallion who’s a lovely match for your mare. You have set money aside for the breeding and associated exams. Soon you’ll simply sit back and wait about 340 days before you can welcome your new arrival, right?

Oh, if only! Mare owners need to consider far more contingencies and have realistic expectations about breeding than simply setting a date and waiting. Pregnancy failure or loss are very real risks, and breeders must take steps to prevent and/or handle such events.

Expect the Unexpected

Mare owners often enter into the breeding game thinking, “This is going to be easy,” says Kristina G. Lu, VMD, Dipl. ACT, of Hagyard Equine Medical Institute, in Lexington, Kentucky. “It will be easy—until it’s not.”

For the majority of mares, says Lu, all does go smoothly. However, subfertile mares and foaling emergencies can lead to emotional roller coasters and mounting expenses for breeders (we discussed a few of these expenses in last month’s cover story).  

“Many owners assume that if they breed a mare, she is pregnant,” says Lloyd Kloppe, DVM, Dipl. ACT, owner of Durango Equine Veterinary Clinic, in Buckeye, Arizona. “But it is not a 100% success system. You should plan on some failure of conception and need to rebreed.”

What’s more, “Owners frequently do not realize that breeding management is not recipe-driven,” adds Margo Macpherson, DVM, Dipl. ACT, professor of Large Animal Clinical Sciences at the University of Florida, Gainesville. “All mares are different, and they respond differently to the varied kinds of semen (fresh, cooled, or frozen-thawed).”

These many variables translate to many reasons why a mare might fail to conceive or lose a foal.

Problems Preventing Pregnancy

Endometritis, an infection or inflammation of the uterus lining, is one of the main reasons why mares fail to conceive, says Dale L. Paccamonti, DVM, Dipl. ACT, professor of theriogenology and head of the Department of Veterinary Clinical Services at Louisiana State University, in Baton Rouge. Endometritis can be either breeding-induced or persistent (chronic).“Some mares experience a significant endometrial inflammatory response to breeding that can interfere with the mare achieving pregnancy,” Lu notes. Some inflammation is natural and necessary to clear the uterus of excess semen and bacteria, but these mares’ reactions are more severe. (In contrast, mares that experience a temporary inflammatory response to mating quickly return to a noninflamed and fertile state.)

Timing of insemination and how long a mare’s egg or stallion’s sperm remains viable can also affect whether a pregnancy takes. “The mare’s ovum or egg starts losing viability six hours after ovulation and has almost zero viability by 24 hours post-ovulation,” Kloppe says. “Sperm viability is highly variable, depending on the stallion. The majority of stallions’ sperm will maintain viability for at least 48 hours, with some lasting for four to seven days. Others may lose viability within hours.”  

Body condition of the mare can play a role in her failing to conceive. Kloppe says it is important that the mare be in good health and at a good weight to support a pregnancy. Veterinarians generally recommend mares score from a 5 to a 6 on the 1-to-9 Henneke Body Condition Score (BCS) scale. Any lower and the mare’s reproductive efficiency begins to decline.

Age affects fertility, with many mares’ egg quality on the decline by age 18. Older mares also have more reproductive conformation-related problems (e.g., a vulva that tilts forward, potentially causing contamination and inflammation of the reproductive tract) that can interfere with a successful pregnancy.

Time of year even figures into conception rates. “During the transitional time (the phase between winter anestrus—not cycling—and regular cyclic ovarian activity, the time of which varies depending on latitude), the mares do not ovulate as reliably as during the breeding season,” Kloppe says.

Early & Late-Term Pregnancy Losses

Even when a mare does get pregnant, there is no assurance she will carry the foal to term. By far, most pregnancy losses occur early, in the first couple of months, says Paccamonti. These might be due to embryonic abnormalities or genetic defects that cause the fetus to be nonviable; defective oocytes; severe illness; and age-related changes in the mare (As the mare ages, the uterus itself is not as healthy as a young mare’s, Kloppe says: “Age-related changes can lead to uterine fibrosis and to a less-efficient placenta. These both make for less nutrition getting to the fetus.”). He adds that severe stress caused by long-distance transport, extreme weather conditions, poor nutrition, and extreme work can be enough to cause a mare to lose her foal.

Uterine infections can also cause mares to lose their foals early in pregnancy. “These can be low-grade infections that were undetected prior to breeding, systemic infections that spread into the placenta, or ascending infections that enter through the vulva and cervix,” Kloppe says.  

Mares can abort closer to term due to conditions such as equine herpesvirus, the presence of twins, or placentitis (inflammation of the placenta). Mares typically don’t encounter many problems mid-term, from about three to eight months, Kloppe notes.

Then there are the mares that carry their foals completely to term, only to lose them at birth. Lu sometimes sees foal loss due to dystocia, which is difficulty giving birth, and associated perinatal asphyxia, the deprivation of oxygen to the foal during birth that lasts long enough to cause physical harm.

Congenital abnormalities in foals are yet another reason for losses after carrying to term, says Lu. “This is a broad category and can include problems such as contracted limbs that are unable to straighten and, thus, lead to dystocia. It can also include abnormally long or twisted umbilical cords,” she says.

While twin pregnancies have historically led to foal loss, they are far less common now that veterinarians have the knowledge and technology (e.g., ultrasonography) to monitor mares early for and reduce twins. “Now we diagnose pregnancy (and twins) by ultrasound between 10 and 15 days, while the embryos are still in the mobility phase,” says Paccamonti, “that’s probably the biggest change. We can successfully reduce twin pregnancy to singleton fairly easily.”

However, if a veterinarian does not make that discovery until much later, the situation becomes more complicated and could result in the loss of both fetuses.

A Checklist for Success

What can a mare owner do to help ensure a successful conception, pregnancy, and foaling? Our sources offered the following suggestions:

  • Research the stallion’s success as a breeding stallion before signing the contract.
  • Ask about the stallion’s fertility with the modality being used. “For instance, if you are breeding with frozen semen, ask about the stallion’s frozen semen pregnancy rate—not just that he has produced foals, though this is useful information,” Lu advises. “Better yet, ask if there is information regarding the pregnancy rate with frozen semen of a similar dose size as yours and frozen around the same time as yours.”
  • Whether the plan is for natural cover or artificial insemination with cooled or frozen-thawed semen, have realistic expectations. “Not all mares get pregnant on one straw of frozen semen,” notes Macpherson.
  • Have a veterinarian examine the mare prior to breeding, and make sure she’s current on vaccinations and deworming.
  • Keep the mare in good body condition. “It is important to have your mare at a proper weight before breeding,” says Kloppe. “Being very underweight (a BCS score of 3 or less) or very overweight (a score of 8 or greater) can reduce pregnancy rates.”
  • If you haven’t done so already, establish a relationship with a veterinarian with whom you communicate well.
  • Ensure prompt veterinary assistance is available if needed. “You might need to take the mare to a clinic for frozen semen breeding,” adds Lu.
  • Check to determine whether the mare needs a Caslick’s, a procedure in which the upper part of the vulva is surgically closed in an effort to reduce contamination of her reproductive system with feces and other environmental contaminants. Mares with poor vulvar conformation might need this procedure performed shortly after conception and the sutures removed two to four weeks before anticipated foaling.
  • Have your veterinarian ultrasound the mare at 14-16 days after breeding to check for twins, and follow up as recommended by your veterinarian.  
  • Schedule a pregnancy check around Day 60 to make sure the mare is still pregnant.
  • Have your veterinarian administer recommended annual core vaccinations (encephalitis, tetanus, etc.) at 10 months of gestation to improve colostrum (the mare’s antibody-rich first milk) quality.
  • Give the pregnant mare some exercise. “Don’t overdo it, of course, but don’t keep her in a stall 24 hours a day,” says Paccamonti.
  • Vaccinate for equine herpesvirus at five, seven, and nine months. Some veterinarians also recommend vaccinating at three months, depending on the region and risk exposure.
  • Assess the mare’s diet and increase or decrease her nutrient intake appropriately for season, housing, and gestational phase.
  • In mid- and late gestation, monitor the mammary glands on the mare’s udder for enlargement or discharge and regurly check the vulva for discharge.
  • If feeding fescue, be sure to remove it as a forage source at 300 days of gestation, as this grass can harbor a toxin that causes late-gestation mares to develop “thick placentas; increased incidence of ‘red bag’—premature separation of the outer placental membrane from the uterine wall that leads to protrusion through the vulva; dystocia; agalactia (lack of milk production); and increased foal death,” Paccamonti says.  
  • Have a plan for monitoring the foaling and a contingency plan for emergencies.
  • Avoid stressing the mare and provide her with a safe place to foal.

Going Forward

Besides the advantages of ultrasonography mentioned, other technologies are making a positive impact on mare conception and pregnancy rates.

The increased use of artificial insemination and minimal-contamination breeding techniques have greatly increased success rates, says Kloppe. Ongoing research is also providing insights into managing conditions such as placentitis and uterine inflammation, notes Lu.

By working together, a responsible owner and knowledgeable veterinarian can foster the best outcome within their powers for a safe, healthy pregnancy and a good start to life for a new foal.


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