Subconsciously, you've been holding your breath for months. From the moment your mare was confirmed in foal, it's been a tense waiting game. And although she will be foaling soon, you know a healthy foal is still anything but a given. Between breeding and her foaling date lurk a few dozen tragic ways in which she could lose her foal. Whether you call it "slipping a foal" or bluntly label it abortion, it's a breeder's most heart-sinking nightmare--and it's all too common.

Want to dodge the bullet? Knowing what could be a risk for your mare, and being able to recognize the warning signs, is half the battle. Following is a run-down of some of the most common ways in which a pregnancy can go wrong.


Nature can terminate a pregnancy for all sorts of reasons. If it happens early in the gestation cycle (in the first 40 days), the tiny embryo is usually expelled through the cervix without notice. According to A.C. Asbury, DVM, Dipl. ACT, Professor Emeritus of the University of Florida and consultant to the Grayson-Jockey Club Research Foundation, an embryo at seven to eight days is only about 0.04 inches (1 mm) in diameter; an embryo at 14 days is about 0.6 inches (15 mm) in diameter. If an embryo this small is slipped, then the owner would likely miss it, he says. Technically, this is called EED, for early embryonic death. EED can be caused by external factors including trauma, stress, or malnutrition; by physiological problems in the mare, such as a hormone deficiency or imbalance or a scarred uterus; by genetic defects in the embryo; or by more than one fetus vying for nutrients. The incidence of EED has been estimated at 5-24%, according to reproductive specialist Jonathan Pycock, BVetMed, PhD, DESM, MRCVS, RCVS, of Equine Reproductive Specialists in Yorkshire, England, and it's most likely to occur in the interval before pregnancy can be detected by ultrasound (before Day 11). If this is the case, the owner might never know that the mare "caught" at all.

Abortion is defined as the situation in which the mare actually delivers a non-viable foal, somewhere between 40 and 300 days. (After 300 days, a living foal which is delivered is premature, but it might have some chance of survival. A dead foal is generally known as a stillbirth). Fetal death can occur at any time during the pregnancy, although it often goes undetected if it happens prior to the four-month mark. Overall, the incidence of abortion in the general equine population is estimated at about 15%, although miniature horses are at higher risk (up to 27%), thanks to their susceptibility to inherited defects, according to Frosty Franklin, DVM, of Edgecliff Equine Hospital in Spokane, Wash.

A late-term abortion (in the last 100 days of the pregnancy) can come with warning signs, or with no signs at all. Breeders should watch mares closely for changes in abdominal shape (indicating an early "dropping" of the fetus as the pelvic muscles relax), premature udder development (sometimes including dripping milk), or vaginal discharge. Sometimes, the first indication of trouble is when you see shreds of placental tissue hanging from your mare's vulva; or a wet, bloody tail that indicates she has already slipped her foal.

Catching abortion before it happens isn't always possible, as the signs are often subtle (and by the time they're noticed, there's often nothing you can do). "There's often very little warning," confirms Terry Morley, DVM, who operates a Standardbred breeding operation near London, Ontario, Canada. "It can happen even to very experienced breeders who check their mares every day."

Abortion doesn't just dash your hopes for a healthy, happy foal. It can also be dangerous for the mother. A fetus which dies in utero is unable to position itself properly in the womb in preparation for birth, so the mare might struggle to expel it. In some cases, veterinary intervention is needed. Once the fetus is delivered, you'll have to make sure the placenta passes within three to four hours at most; if it doesn't, the mare is at serious risk of infection and laminitis.

Veterinarians have identified many causes of abortion, but most cases remain undiagnosed. Only about 40% of equine abortions are ever assigned a definite cause through laboratory analysis, says Bob Wright, DVM, of the Ontario Ministry of Agriculture, Food, and Rural Affairs' Animal Industry Branch in Ontario. If you're unlucky enough to have an aborted foal, it's important to submit it--and the placenta--to a laboratory as soon as possible for analysis, especially if you have other broodmares on the property. If the cause is infectious, you'll want to make management changes to protect other foals you're expecting. Even if it's noninfectious, a necropsy could provide you with valuable information to help you prevent a repeat tragedy.

It's important to keep in mind that stress, either physical (such as colic) or psychological (such as weaning the foal-at-foot or long-distance shipping), can predispose a mare to abortion. The security of a late-term pregnancy depends on many factors, including the health and vitality of the placenta, the integrity of its attachment to the uterus, the maintenance of the cervical seal, and the mare's blood pressure and blood oxygen saturation. The placenta is quite pivotal, as we'll see--and as the fetus grows, the demands placed on it, both to nourish and to efficiently eliminate wastes, multiply.

The Role of Progesterone

For the first two weeks of pregnancy, the hormones produced by the mare are the same as those she normally has when she is "open" and in diestrus (out of season). But that all changes around Day 14. In an open mare, the hormone called prostaglandin destroys the corpus luteum (CL, a secretory gland formed after ovulation from the tissues of the ruptured ovarian follicle). In a pregnant mare, the CL persists and continues to secrete progesterone, the hormone which prevents the mare from coming back into heat; tightens the seal of the cervix (the entrance to the uterus) to prevent infection; and causes the uterus to become increasingly vascular (blood-engorged) to help nourish the unborn foal.

Horses are fairly unique in terms of how progesterone continues to be secreted to maintain the pregnancy. In the base of the pregnant horn of the mare's uterus, distinctive, irregularly-shaped areas develop in a ring configuration; these are called the "endometrial cups," and they secrete a special hormone called equine chorionic gonadotropin (eCG), detectable in the bloodstream by about Day 37 of the pregnancy (and usually persisting until about Day 120 or more). Only a few other species are known to use this mechanism, including elephants and rhesus monkeys.

The role of eCG is to maintain and develop the CL beyond Day 37, and trigger further corpora lutea to develop (also called secondary or accessory CLs). Meanwhile, progesterone secreted by those CLs keeps the mare out of heat. Between Days 70 and 100, the endometrial cups start to shrink in size, and eventually detach from the uterine wall and slough off, while the placenta takes over the job of progesterone secretion. After the fourth or fifth month, progesterone levels gradually decline, until they reach a low of about 2 ng/mL by about Day 180 of the pregnancy. According to Mats Troedsson, DVM, PhD, Dipl. ACT, service chief of the reproduction department and professor in the Department of Large Animal Clinical Sciences at the University of Florida, progesterone continues to be produced by the fetal placental unit, but metabolizes quickly once it reaches the maternal circulation-- hence, the low detectable progesterone levels. The pregnancy still needs progesterone to be maintained. The role of metabolites, or building blocks of progesterone (if any), and the shift of them at the time of parturition is an area of sparse understanding.

Progesterone is critical for maintenance of an equine pregnancy, especially in the beginning stages, and it has been suggested that a progesterone deficiency might be responsible for many cases of EED or abortion. In theory, since the mare's system of progesterone production is more complex than in most species, there's a greater likelihood of something going wrong. The most likely explanation for a progesterone deficiency would be a malfunction of the corpus luteum, says Troedsson. However, very little research in this area has been completed (see article #3929 at Secondary luteal failure as a result of endotoxemia, which causes prostaglandin release and destruction of the CL, was demonstrated several years ago.

Because progesterone levels are so crucial to a healthy pregnancy, many veterinarians recommend the administration of extra progesterone in the form of the drug altrenogest (Regumate) as a preventive measure for mares which are suspected of being difficult to keep in foal. This therapy is something of a matter of debate, with some practitioners considering it a wise precaution, and others seeing it having questionable benefit.

Bob Boyce, DVM, who runs a busy reproductive practice at the Ilderton Equine Clinic in Ilderton, Ontario, Canada, notes that Regumate is usually administered for the first third of the pregnancy. After that, the mare can be gradually weaned off the drug, although in exceptional circumstances--such as a valuable mare with a history of habitual abortion--Boyce says the mare might be maintained on Regumate through almost her entire pregnancy. While administering the drug is inconvenient (it requires wearing gloves, and some mares resist their daily doses), it has no notable side-effects on either mare or foal, nor does it affect the mare's future fertility.

Troedsson says that up to day 80-100, the pregnancy is dependent on progesterone from the CL, and a healthy pregnancy could theoretically be lost because of an unrelated CL problem. After this time, however, the progesterone is produced by a viable fetus and placenta. If the fetal placental unit can't produce enough progesterone to maintain the pregnancy, the problem is usually beyond help with simple supplementation. However, he says there is some evidence that doubling the Regumate dose could help. He emphasizes that there is a lot of controversy and lack of knowledge about progesterone supplementation.

Regumate is not cheap, so the decision to use it should be carefully considered.

Infectious Abortion

Microscopic invaders are responsible for a large proportion of equine abortions, either as a direct effect or as an "aftershock" from a traumatic disease. Bacteria, such as Salmonella, Streptococcus, Actinobacillus, Corynebacterium, Klebsiella, and Pseudomonas spp. (just to name a few of the bacteria known to trigger abortion in certain circumstances), can enter the reproductive tract of the mare during foaling or breeding, and travel to the uterus, where they infect the fetal membranes and kill the fetus. Viral causes of abortion are even more numerous. Here are the most important:

Equine Herpesvirus (EHV-1)--Formerly called rhinopneumonitis, EHV-1 has long been recognized as a major cause of "abortion storms" that can sweep through a broodmare band and cause losses approaching 100% of the fetuses with few or no warning signs, says Wright. These abortion storms can occur weeks or months after the mares are infected. It's only in the last couple of decades, though, that we've separated the strain of equine herpesvirus that triggers flu-like respiratory disease (EHV-4) from the strain that is primarily responsible for abortion (EHV-1). (EHV-4 can cause abortion, but it tends to trigger individual cases rather than storms.)

EHV-1 usually triggers abortion at seven to 10 months of gestation. On necropsy, a fetus aborted because of EHV-1 before six months of gestation will usually have widespread cell necrosis (death); older ones will demonstrate signs of jaundice, excess pleural fluid (fluid in the lung), pulmonary edema (fluid swelling), liver damage, and an enlarged spleen. The fetus and placenta are extremely infectious, so contact with other mares should be avoided so that the virus's spread is minimized.

While EHV-1 is one of the most significant viral forms of abortion, the good news is that it's largely preventable. New EHV-1 vaccines have a much higher efficacy rating than in the past, and they can be safely used to protect mares from the disease. Begin the vaccination series three to four weeks before your mare is bred, and give boosters at five, seven, and nine months of gestation for the best degree of protection, says Asbury. Some isolated cases of abortion can still occur in vaccinated mares, but it prevents abortion storms.

Leptospirosis--Nicknamed "lepto," this disease begins with transient flu-like symptoms (mild depression, loss of appetite, and a fever) that last about two to four days. These symptoms often go unnoticed if your mares are outside all the time. Up to four weeks later, abortion occurs, at just about any time in the second half of the pregnancy. (Necropsy reveals disease lesions scattered randomly over the placenta, leading to placentitis--which we'll discuss in a minute.) Occasionally, foals are born alive, but they are generally so weak that they quickly perish (though aggressive antibiotic therapy can sometimes save the day).

Leptospira bacteria are spread through urine (infected animals can shed the micro-organism for months after exposure) and indirectly through contaminated feed and water. Rodents are the major carriers, so it's essential to make feed containers rodent-proof and use barn cats to keep the vermin population low. Leptospira can also be transmitted from cattle and wild ruminants such as deer and elk. Wet environmental conditions seem to increase the risk.

There's little you can do to detect leptospirosis ahead of time; a rising lepto titer in the blood is diagnostic, but blood samples must be taken on a frequent basis to catch it. If you do happen to identify the disease, treating the mare with antibiotics such as penicillin might be successful in preventing abortion. It's crucial to isolate any mares which have aborted, and thoroughly disinfect their stalls and any equipment that has come in contact with them. Keep in mind that cattle vaccines are ineffective in horses.

Potomac Horse Fever (PHF)--Most of us know that Neorickettsia risticii (formerly known as Ehrlichia risticii), the causative organism of Potomac horse fever, generally triggers symptoms such as projectile diarrhea (resulting in severe dehydration). But N. risticii has also been identified as a cause of equine abortion on rare occasion, either by crossing the placenta and infecting the fetus, or by stressing the mare and causing the cervix to soften inappropriately so that the uterine environment becomes infected by an opportunistic organism. Pregnant mares which have had PHF are also at risk for retaining the placenta when they foal.

There is a vaccine designed to combat PHF, and many veterinarians recommend administering it to in-foal mares, although it doesn't provide 100% protection and some veterinarians are skeptical of using it.

Equine Viral Arteritis (EVA)--This venereal disease, usually transmitted to the mare by a stallion, can cause clinical signs including a high fever, snotty nose, diarrhea, and edema of the legs and the ventral line of the abdomen. However, it might cause no visible signs at all. Certain strains of EVA are known to cause abortion one to three weeks after exposure (not weeks or months later as with most other viral forms of abortion). Since EVA can be spread through respiratory contact, it's essential to isolate affected horses. Only breed mares which are properly vaccinated and managed to stallions which are EVA carriers.

Contagious Equine Metritis (CEM)--Taylorella equigenitalis, the highly contagious virus that causes CEM, is also transmitted sexually; infected mares are often asymptomatic. Sometimes a profuse, mucopurulent (containing mucus and pus) discharge from the vulva can be seen. Foals of CEM mares might be infected at birth; abortion isn't common with this disease, but it's possible if the infection is severe enough. Failure to get back in foal immediately can occur, but there's a good prognosis for future fertility. Prevention is the only way to dodge this bullet.

The Perils of Placentitis

By far, the most common cause of abortion, accounting for some 30% of diagnosed cases, is placentitis (inflammation of the placenta). Placental insufficiency is a wider term including both infectious and non-infectious conditions. When there's placental insufficiency, mares either abort or produce a growth-restricted baby which might never catch up with peers. Despite significant advances in herd health management over the past few decades, placentitis is something we haven't yet solved. Its incidence has not decreased, and it's still considered a major problem facing the breeding industry.

Placentitis is an individual condition; it usually doesn't sweep through a herd. Fortunately, mares which abort from placentitis rarely remain infected if correctly treated; they can usually be bred back soon after they slip a foal, and they still have a chance at a normal baby the following year.

Researchers have identified three main types of placentitis:

Cervical placentitis occurs when a bacterial or fungal infection enters at the cervical star (the point where the placenta contacts the cervix) and spreads anteriorly (forward), distorting the placenta's connection with the uterus and compromising its ability to nourish the fetus. Sometimes the infection (which can also enter the reproductive system through the bloodstream) infects the fetus by entering the placental circulation.

Diffuse placentitis is caused by a blood-borne agent such Leptospira. In diffuse placentitis, disease lesions appear randomly all over the surface of the placenta, compromising its ability to deliver nutrients and sweep away wastes from the fetus.

Placentitis occurring at the base of the placental horns usually has an unknown cause. Researchers suspect that the causative organism might enter through the cervix when the mare is bred. An organism that has not been fully classified, but appears to be similar to a bacterium called Nocardia (it's Gram-positive, spore-forming, and has branching rods), seems to be at the heart of this mysterious form of placentitis, which is on the rise in some parts of North America. Once established, the organism initiates partial placental separation from the uterus, depriving the fetus of nourishment; if the mare doesn't abort, the foal will be born a month or more premature, and it will be small and weak.

With nocardioform placentitis (often caused by Crossiella equi bacteria), the mare will often show premature mammary development (her udder will fill with milk). Since the infection starts at the base of a uterine horn and rarely involves the cervix, there's generally no cervical discharge. A transabdominal ultrasound can often detect the degree and severity of the infection; it will reveal placental separation and exudate on the ventral (lower) placental surface. Asbury says that post-foaling, the placenta looks markedly abnormal. Most changes are seen on the chorionic side of the membranes (the side which interfaces the uterus). The entire placenta might be thickened, or the problem might be localized. Dark brown to mud-colored exudates will appear on the surface of the placenta in the area affected by the inflammation. These exudates are typically thick, tenacious substances that cling to the placental wall.

The only good news about nocardioform placentitis is that mares tend to clear this organism from their systems quickly after abortion, and they rarely need antibiotic therapy. It also causes little reaction or scarring of the uterus. Because the mode of entry is suspected to be the cervix, there are a couple of surgical options which might reduce the risk--one is the traditional Caslick's operation, which sutures the upper part of the vulvar lips to help keep infection out, and the other is urethral extension, which brings the urethral opening closer to the vulvar lips to avoid urine entering the reproductive tract.

Forms of cervical placentitis can be diagnosed with a transrectal ultrasound, which allows your veterinarian to measure the thickness of the placenta. An increase in thickness indicates swelling and inflammation, which are possible tip-offs to an impending abortion. If you manage to detect placentitis in time, your veterinarian can place your mare on a regimen of antibiotics, progesterone, and uterine anti-contraction medications to try to prevent disaster. This routine, however, has mixed results. The outcome depends on the degree of placental insult, which is difficult to predict clinically, says Troedsson.

Non-Infectious Causes

Just as numerous as the viral and bacterial causes of abortion are the physical problems or defects that can make it impossible for a mare to maintain pregnancy. Simple glitches such as umbilical cord torsion (twists in the cord that cut off the flow of nutrients to the fetus) account for some fetal deaths, while congenital defects that make the unborn foal non-viable are responsible for an unknown number of others. (Severe fetal abnormalities are usually terminated early by Mother Nature, but some--such as lethal white syndrome in homozygous overo Paints--might be aborted late or even carried to term.)

The leading non-infectious cause of equine abortion is twinning, the scenario in which two fetuses are conceived and both manage to anchor themselves in the uterus. Horses are poorly adapted to support more than one embryo. Generally, the placenta just can't deliver enough nutrients to nourish both. The result is two undersized fetuses, neither of which survive to term--or one normal-sized foal and a weak runt with a poor chance of survival.

Because twin pregnancies have such a poor chance of success, most veterinarians recommend ultrasound exams of the uterus at about Day 11 to detect the presence of more than one embryo. Eliminating one twin by crushing or pinching it at 15 days is a simple procedure that can be done at the farm. The remaining embryo will then grow to term as a normal, full-sized foal. If pinching isn't done, the mare's system might eliminate one embryo naturally, but if that doesn't happen by about 35 days or so, chances are she'll abort both fetuses sometime in mid- to late gestation.

The incidence of twinning varies quite a bit from breed to breed, with larger mares (Thoroughbreds and draft breeds, in particular) at greater risk than smaller ones. It's almost unheard of in most pony breeds.

After the 42-day mark, breeders can generally breathe a little more easily if the ultrasounds confirm a single, healthy embryo. Periodic monitoring, either with ultrasound or manual palpation, doesn't hurt. Boyce likes to re-check mares at about 60 days, once later in the fall, and at least once during the winter.

"The period between 21 and 42 days is the highest-risk time for early embryonic mortality," he says.

Premature separation of the placenta is a typical finding in mares that are in the process of abortion, and in the occasional full-term delivery, says Asbury. The interface between uterus and placenta is made up of thousands of minute junctions, called microcotyledons (see "Placental Attachment" on page 31). Each one is a site for exchange of nutrients and oxygen moving from mare to fetus. In a normal delivery, the placenta ruptures at the cervix, allowing the foal to exit the birth canal, while the majority of the attachments remain in place throughout the birth process and beyond. The subsequent release of the connections allows the placenta to be delivered.

When separation occurs prematurely, the fetus is deprived of oxygen and will survive for only a short period, whether delivered or not. If the foal is near maturity, detecting placental separation might help you save the foal by expediting delivery. A typical example is the "red bag" delivery. In this situation, the placenta separates away from the uterus in the region of the cervix. That results in intact membranes being delivered without rupturing and allowing release of the foal. If this separation is recognized early, the membranes can be cut or otherwise opened to release the foal.

Placental separation precedes abortion and can be recognized by ultrasound examination of the junction between the uterus and placenta in the area just in front of the cervix. If the process is progressing, there might be a noticeable discharge from the vulva, or rarely, evidence of bleeding.

Other causes of abortion exist--such as ingestion of toxic plants, malnourishment, vitamin or mineral deficiencies, and overdosing on certain medications. And, as is the case with mare reproductive loss syndrome in 2001 and 2002 in Ohio, Kentucky, and surrounding states, sometimes the cause of abortion is not fully understood.

Even if you watch your broodmare every second, you can't protect her from all of the possible invaders in her environment, and from within. Like breeders everywhere, all you can do is take every sensible precaution, ensure she's in the best possible health when you start the breeding process, feed her properly, give her proper veterinary care, and cross your fingers. Here's to a healthy foal. 


See the Abortion, Embryonic Death, Mare Reproductive Loss (MRLS), and Mare Care/Problems sections under Breeding/Reproduction at

About the Author

Karen Briggs

Karen Briggs is the author of six books, including the recently updated Understanding Equine Nutrition as well as Understanding The Pony, both published by Eclipse Press. She's written a few thousand articles on subjects ranging from guttural pouch infections to how to compost your manure. She is also a Canadian certified riding coach, an equine nutritionist, and works in media relations for the harness racing industry. She lives with her band of off-the-track Thoroughbreds on a farm near Guelph, Ontario, and dabbles in eventing.

Stay on top of the most recent Horse Health news with FREE weekly newsletters from Learn More

Free Newsletters

Sign up for the latest in:

From our partners