6 high-risk scenarios to watch for in pregnant mares

It’s a crisp night in February, 2 a.m., and large flakes of snow are fluttering like shavings in the hazy moonlight over the National Stud Breeding Center grounds in Neustadt, northeastern Germany. Bundled in heavy work coats and snow-dusted scarves and hats, staff members crunch along a white path in their wool-lined boots toward a brick building and open its door. There, in long rows of straw-filled stalls in a dimly lit hall, lie dozens of sleeping—and snoring—pregnant Warmblood mares.

It’s too early in the season for foaling. In fact, at this stage a foal is the last thing the staff hopes to see, as it would mean a mare had aborted—and, worse, that they’d missed the signs. That’s exactly why they’re here now, in the middle of a dark, freezing night. They’re looking for signs of complications and just about anything that might seem unusual in these precious mares, who make up not only part of the government ­breeding program but also the Graf Lehndorff Institute for Equine Science’s research herd.

Their visit is part of a well-established routine designed to detect risky pregnancies as early as possible. Such careful monitoring is key for safeguarding the life of mare and foal, and our sources say it’s something every breeder should be doing in every broodmare barn.

To help you identify at-risk pregnant mares, we’ve compiled a list of six focal points for your regular routine checks. But before we get started, let’s learn why such careful monitoring matters. 

Surveillance and Record-Keeping

A popular misconception in the horse breeding world is that abortions can’t be stopped. On the contrary, handlers can help prevent fetal loss—and ensure mare health—by watching mares closely throughout gestation for early signs of problems, says Christine Aurich, DVM, PhD, head of the Graf Lehndorff Institute.

It’s the early part that’s critical, explains Margo Macpherson, DVM, MS, Dipl. ACT, professor of large animal reproduction at the University of Florida’s College of Veterinary Medicine, in Gainesville. Disease “starts insidiously in most cases,” she says, and by the time signs become evident, the illness might have progressed to a point that treatment is less efficient. “Early intervention is the best means of saving a pregnancy,” Macpherson says.

Paying attention to your mare’s physical condition and behavior every day allows you recognize what’s normal for her, so you quickly recognize what’s not normal, she adds. Keeping written records can help you and your veterinarian notice and track irregularities. “Information is powerful,” says Macpherson. “Vigilance and observation are the most important tools you have.”

Armed with that knowledge, let’s look at things you can watch for as a vigilant broodmare owner or manager:

1. Abnormal ultrasound findings

Ultrasound exams are your veterinarian’s job, of course. But it’s up to you to schedule them regularly, because they’re critical for finding problems you can’t see—especially early in the pregnancy.

Once such problem is a twin pregnancy. The vast majority of twins die before birth—at any time of gestation—but ultrasound at Day 14 or 15 after ovulation allows veterinarians to diagnose twins and reduce one embryo, leading to a 95% chance of a healthy foal at term, says Aurich. 

Ultrasound exams at 40 days and five to six months after ovulation, and measuring the combined thickness of the uterus and placenta (CTUP) and fetal heart rate any time the mare shows signs of impending abortion can help the practitioner pick up on problems with the placenta, such as placentitis (see below), or even that the mare’s quietly aborted.

2. Unusual discharge from the vulva

Simply checking the mare’s vulva every day can quickly alert handlers to one of the most common reasons for pregnancy loss, says Macpherson. Placentitis—an infection of the placenta—causes 30% of abortions in U.S. horses. Most cases are caused by bacteria entering the reproductive tract and creeping into the uterus through the cervix to attack the ­placenta—aptly called ascending placentitis. Fungi and viruses are occasional culprits, as well.

Although placentitis can occur throughout gestation, it’s most common after seven months, Aurich explains.

Purulent (containing pus) discharge from the vulva is a red flag, Macpherson says. But any discharge is worth reporting to your veterinarian so he or she can measure CTUP.

Fortunately, observant owners easily recognize abnormal. “If they’re looking every day, they’re going to know when something’s different,” she says. Even if the mare has a bloody discharge—typically harmless and caused by broken varicose veins in the vagina—owners should let their vets know.

Rapid intervention with antibiotics and anti-inflammatories to treat placentitis can save pregnancies, says Macpherson, citing her research data.

3. Early mammary gland development

A telltale sign of a pregnancy in trouble is premature mammary gland (udder) development. In a healthy pregnancy the glands start developing two to four weeks before delivery, says Macpherson. Anything sooner is cause for concern.

The same hormonal signals that trigger foaling trigger udder development (so the mare can feed her newborn), she says. If that happens more than a month before term, veterinarians must intervene. “The pregnancy is trying to terminate itself,” she says. 

Abortion might happen because of twins—which occasionally get missed on ultrasound, says Aurich. But multiple other conditions, ranging from genetic abnormalities of the foal to structural problems in the reproductive tract, can spur fetal loss. In certain situations, such as an infected wound releasing high concentrations of abortion-inducing inflammatory mediators, veterinarians can stop the process before it’s too late.

4. Local infectious disease outbreaks

Outbreaks of abortive viral and bacterial diseases such as equine herpesviruses 1 and 4 (EHV-1 and -4), equine viral arteritis (EVA), and leptospirosis can push entire herds of pregnant mares into a high-risk category, regardless of gestational stage, our sources say.

Infectious respiratory diseases not specifically associated with abortion can also lead to pregnancy loss by compromising the mare’s breathing. “Pregnancy exerts a lot of stresses on the bodily systems, and the respiratory tract is one of them,” says Macpherson. “If the mare can’t breathe properly, this presents a risk for the pregnancy.”

Breeders can help protect their mares by following strict vaccination schedules, says Aurich. In particular, EHV-1 and EHV-4 vaccines provide good—but not absolute—protection from infection. Equine-herpesvirus-related abortions usually occur after seven months’ gestation, she adds.

Implementing biosecurity measures is critical—always, not just during ­outbreaks—and should include isolating pregnant mares from other horses, especially equine athletes with frequent exposure to other horses, says Aurich.

Keeping stable groups of mares reduces risks of introducing pathogens (­disease-causing organisms), adds Macpherson. And handlers should practice good hygiene, washing hands and equipment thoroughly between handling horses—“a simple and effective way to stop spread of disease,” she says.

5. Signs of colic or general illness

Colic can be life-threatening in any horse, but in a broodmare it threatens two lives. It’s also exacerbated and sometimes sparked by the presence of the growing fetus close to the mare’s digestive tract, says Macpherson. “It displaces the bowel and can cause it to have significant torsion (twisting),” she says. The weight of the fetus can even, in rare cases, cause necrosis (cell death) of the intestines, causing the gut to “leak” into the abdomen and establish a dangerous infection.

Then there are the systemic illnesses—infections and fevers affecting the entire body—that push the pregnancy into a higher risk category. Aurich notes that greater demands on the mare’s body, especially late in pregnancy when the foal is growing quickly, make it harder for her to fight off disease.

“In mares with colic or systemic illness, inflammatory factors may be released in high concentrations, compromise the pregnancy, and result in abortion,” she says.

Veterinarians can reduce the risk of this happening by treating with non-steroidal anti-inflammatories—but never corticosteroids, she adds, which can induce foaling/abortion, especially if they’re given repeatedly.

6. Strange behavior

One of the most important signs of a high-risk pregnancy is a subtle change in the mare’s behavior, says Macpherson. Attentive handlers who spend time observing the mare daily notice these early signs best, she says.

These include changes in appetite—especially refusing to eat, says Macpherson. But less obvious signs include standing away from the rest of the herd, lying down more frequently than usual, or showing behavioral signs of pain. Familiarity with the mare’s regular habits can help distinguish between mild but normal discomfort that can occur in late gestation and more serious conditions.

Handlers who vigilantly observe mare behavior can catch high-risk issues such as endotoxemia (toxins in the bloodstream), hyperlipidemia (high fat concentrations in the blood), uterine torsion, umbilical cord torsion, prepubic tendon rupture, and placentitis, all of which make the mare act unwell, Aurich says. Many of these issues can improve with early treatment. Our sources agree that observing pregnant mares day and night isn’t always feasible, of course, especially on small farms. Breeders can consider complementing their physical presence with the use of technology such as cameras and wearable equipment like “smart halters.” 

Before You Breed, Is Your Mare High-Risk?

Good surveillance begins before the pregnancy. Know your mare’s history, our sources say, to determine whether she’s considered high-risk from the start.

Mares that have already had difficult pregnancies or deliveries are often high-risk for repeat issues, says Macpherson. For example, if she hemorrhaged during foaling, she might have ruptured a major blood vessel. This is more common in older mares and might indicate the vessel is damaged.

If the mare previously aborted a dead foal, experienced dystocia (difficult delivery), and/or had retained fetal membranes, she could be less likely to conceive and, if she does, have a greater risk of complications, Aurich says.

Abortions due to genetic problems such as Warmblood fragile foal syndrome, overo lethal white syndrome in Paint Horses, and severe combined immunodeficiency in Arabians could indicate the mare is a carrier for the disorder, leading to more abortions, she says. Genetic testing could help inform more successful breeding choices, especially in selecting a compatible stallion.

Twinning “most probably has a genetic component,” which is becoming more prevalent in breeding stock now that a twin can survive after early embryo reduction and go on to reproduce, Aurich says. So, for example, a mare that had twins is more likely to have them again and needs careful ultrasound evaluation in early gestation.

Placentitis can be a repeat problem when the perineum and/or vulva has structural abnormalities that make it easier for bacteria to enter the reproductive tract, says Aurich. A breeding soundness examination, including endometrial bacterial culture and cytology, can help detect at-risk mares. Veterinarians can then treat them during pregnancy with antibiotics and, if warranted, a Caslick’s procedure to partially close the vulva. They’re also good candidates for embryo transfer to another mare, she adds.

For all pregnancies, but especially high-risk ones, owners should get their veterinarians involved early and maintain good working relationships with them as a team, says Macpherson. Many clinics, including the University of Florida where she works, offer flat-rate veterinary care packages for gestation so owners can benefit from regular visits without worrying about itemized expenses. “This isn’t a time to cut corners,” she says—particularly when risks are high.

Take-Home Message

Serious complications are uncommon in equine pregnancies, but when they occur the consequences can be devastating for both mare and foal. Breeders can reduce risks by monitoring mares closely and recognizing problems early. When you have a high-risk pregnant mare and understand the dangers involved, you can provide her with the best supervision and veterinary care to help ensure her success.