Pregnancy loss in the mare

Any loss of a pregnancy before 30 days of age is referred to as early embryonic loss. The term abortion is used to describe death of a foetus between 30 and 300 days. This is because it is deemed that a foal is not viable under 300 days of age. In reality, any foal born before 310-320 days of gestation is unlikely to survive. Even with intensive care, internal organs such as the lungs are underdeveloped and the cuboidal bones of the hocks and knees are not likely to be sufficiently ossified (hardened) to support the weight of the foal. The term stillborn is used to refer to any pregnancy lost between the period of 300 and 340 days of gestation (the historical ‘average’ length of gestation in a mare). It does not pertain to any foal that is born alive, but is subsequently found dead.

Pregnancy loss is not an uncommon event, with an expected normal rate of loss of up to 5% between day 16 and day 28 of gestation. Another 5% may be lost between day 28 and day 45, emphasising the importance of these early ultrasound pregnancy examinations. Losses after 45 days have a tendency to occur late in the pregnancy, typically after 5 months. Older mares are over-represented as are those mares with a history of reproductive loss.

While many mares will simply ‘slip’ their foal early in gestation, leaving the owner to find the foetus in the paddock, mares aborting later in their pregnancy may need assistance. The degree of warning that a mare may abort is variable. Signs of colic or mild discomfort may be all that are noted. Importantly, although the underlying cause of the abortion may have been at work for some time, the mare will likely be unprepared both hormonally and physically. In the absence of a live foal to trigger the normal process of labour, the cervix may still be closed and the pelvic organs not yet relaxed. The foetus is often in an unusual position and may not present for delivery in the normal manner. Mares with a Caslick may encounter some trauma to the vulva if they abort before the sutures have been removed.

Mares further along in their gestation, with an impending stillborn, typically provide the owner with some suspicion that all is not proceeding healthily. Clinical signs such as bagging up and or running milk prematurely, or visible vulval discharge may be noted (see information sheet on placentitis). A veterinarian should examine the mare and perform an ultrasound to assess foetal viability and determine if the pregnancy is salvageable with institution of appropriate therapy. The pregnancy will be regarded as high risk in such a case and require intensive monitoring. If a live foal is born, the chance of sepsis remains high, but (depending on the extent of any infection) with appropriate attention and therapy, the prognosis for developing into a normal, healthy foal is good.


Bacterial Infection

Most result in placentitis, others lead to premature placental separation. A predisposition to such infection regularly exists because of poor reproductive conformation or prior trauma to the reproductive tract. As there are many surgical repair procedures available to improve the mare’s conformation and therefore her chances of carrying a foal to term, a thorough pre-breeding examination of the mare is advisable. A Caslick’s or Pouret’s procedure should be performed if indicated.

Common bacterial agents implicated in placentitis are:

Streptococcus – thickening of the placenta may be noted at the site of infection. Once the infection is chronic, the placenta is unable to adequately nourish the foetus, resulting in abortion at around 3-6 months of gestation. Grey, thickened patches may be noted on the placenta.

E.coli – red/brown discolouration of the placenta may be noted.

Klebsiella – this organism is also implicated in subfertility in mares and can be difficult to successfully treat.

Pseudomonas – the cervical star (a section of the placenta) may be dark red, while the allantois (the inner sac of the placenta) may be grey rather than white

Fungal Infection

Sometimes distinguished by a thickened placenta covered in brown plaques, fungal infections commonly cause placentitis, and often result in abortion. Aborted foetuses may be undersized.

Viral Infection

Equine Herpes Virus 1 (EHV-1) – this highly contagious virus spreads by ingestion, inhalation or direct contact with viral particles shed in abortion, saliva or nasal discharge etc. In affected mares, it attacks the foetus via the placenta. Abortion usually occurs 3-12 weeks later. Abortions typically occur in late gestation i.e. 8 months onward, but they can occur earlier. Mares often show no signs of impending abortion.

As the virus can adopt a state of dormancy in the horse after initial infection and then reappear in times of stress, new outbreaks are possible amongst susceptible mares. If an abortion occurs in a paddock containing a group of mares, that group needs to be quarantined from other mares on the property. Owing to the inquisitive nature of mares and their tendency to gather around an aborted foetus, mares are often already infected with EHV-1 before the foetus is found.

Virus may be shed by clinically affected and carrier horses for 3 weeks or more, and EHV-1 may remain infective in the environment for up to 14 days and on horse hair for 35-42 days.

Thankfully, with widespread vaccination of mares at months 5, 7 and 9 of gestation, abortion storms caused by EHV-1 are now much less commonplace.

Contagious Equine Metritis – an uncommon cause of abortion. CEM is more commonly associated with failure to conceive in the sub-fertile mare.

Equine Viral Arteritis – though the virus is present in Australia, abortion due to this virus is unlikely.

Placental Abnormalities

Umbilical cord torsion and cord length – while the umbilical cord naturally has twists to it, it should not have any swelling or obvious discolouration. Shorter cords than normal are prone to premature rupture, while extra-long cords can twist excessively, compromising flow to the foetus.

Placental Insufficiency – incomplete development of the villi (the finger-like projections on the placental surface that supply nutrients) often results from infection and may lead to premature placental separation.


This term refers to man-made causes of pregnancy loss, for instance where the wrong mare has been given PG or oxytocin injections, where cortisone has been given to a mare for an allergic condition or where a wormer containing organic phosphates has been used.


Low progesterone levels are an uncommon cause of abortion. As loss of the pregnancy generally occurs at less than 100 days of gestation, it is often undiagnosed.

Reproductive Tract Incompetence

Trauma to the uterus can lead to scarring and inhibit the ability of the placenta to supply nutrients to the foetus. An incompetent cervix and or poor perineal tone are also capable of leading to foetal loss. Embryo transfer is sometimes a better proposition for these mares, as getting in foal is not as much of a problem as the uterus not being a good environment for the developing foetus.

Foetal Abnormalities

Chromosomal defects generally lead to pregnancy loss before 100 days of gestation. Other deformities in the skeleton, the internal organs or the immune system can similarly lead to loss of a pregnancy. However, not all deformed foetuses will abort, some are born alive and need to be euthanised on humane grounds shortly thereafter. Examples include severe wry nose and severe cleft palate abnormalities.


The Eastern Tent Caterpillar has been implicated in abortion in some areas of Australia. Also of importance are some toxic weeds and grasses linked with abortion, along with insecticides and weed poisons that should be used carefully around pregnant mares.

Embryonic location or multiple pregnancies:

Fundic pregnancy – this is the term used where instead of implanting in one horn of the uterus, the pregnancy implants either in the body of the uterus or at the junction of the uterus and the horns. In this location, there is insufficient space for the foetus to continue growing.

Twins – most twin pregnancies will spontaneously abort. In the event the pregnancies survive to term, at least one, if not both foals will be undersized and may not be viable. Twin pregnancies are far less common now, owing to the widespread use of ultrasound scanning in early pregnancy.

Illness, Fever, Pain and Stress

The presence of a fever in the mare can be harmful to the developing foetus, which is not capable of coping well with temperatures outside the normal range. Equally the presence of endotoxaemia (e.g. due to acute diarrhoea, colitis or travel sickness) in the mare can cause abortion of a developing pregnancy. Physical trauma is a less common cause of abortion, the fluid uterus protects the foetus within well. The impact of emotional stress and physical pain of the mare on a pregnancy is difficult to quantify but may be a contributing factor in some abortions – if high levels of cortisone are being produced by the mare over a reasonable duration of time. Short term pain in the form of a foot abscess or laceration appears to be well tolerated by the pregnant mare.


Pregnant mares need to be fed appropriately. Malnourished mares may abort in order to ensure their own survival. Underweight mares may also develop characteristic sunken conformation around the vulva and anus, predisposing them to ascending infections and placentitis.