As foaling season approaches, breeders are highly anticipating their new arrivals. Whether you are breeding your mare for the first time or operating a large breeding organization, every foaling is just as magical and exciting as the first time you witness it.

But why do some mares not carry their foals to full term? Why are some foals born with abnormalities?

Causes of abortion in mares can include bacterial or viral infections, hormonal imbalances in the mare, twinning and congenital abnormalities in the foal. The most common cause of viral abortion is Equine Herpes Virus (EHV). EHV has different strains that can cause different symptoms in horses, but the strain that causes abortion is EHV-1. If infected with EHV-1, mares often abort between 8 and 11 months’ gestation, although some carry to term and the foal dies shortly after birth. The good news is that there is a vaccine available to prevent this disease in mares; however, while the efficacy of the vaccine is questionable, it is still recommended. The easiest way to determine the cause of infectious abortion is to submit the fetus for testing at your local lab, where a series of tests are conducted to determine the cause.

There are also noninfectious causes of abortion. One of the main culprits is a twin pregnancy. The equine placenta is not designed to provide enough nutrients for two developing foals; this results in fetal death due to not enough nutritional support. Thankfully, ultrasound technology has made it possible to identify a twin pregnancy early and terminate one fetus, allowing for the other fetus to be carried to full term.

Progesterone deficiency is another cause of abortion. Progesterone is the hormone produced chiefly by the ovaries (specifically the corpus luteum) and is responsible for preparing the uterus for pregnancy. After 40-50 days, the uterus has formed endometrial cups, which produce a prostaglandin that stimulates the ovaries to produce more corpora lutea used to maintain pregnancy until the 4th or 5th month of pregnancy. After that, the placenta itself is responsible for producing enough progesterone to maintain pregnancy. There are theories that low levels of progesterone produced by the placenta cause abortion. Thankfully, there is an oral progesterone supplement available to increase progesterone levels in the body that will improve the chances of carrying the foal to full term.

Congenital (present at birth) abnormalities can cause abortion in mares if the developmental problem is severe enough, but can also occur in foals that are carried to full term. These abnormalities are often related to problems with tissue development, whereas genetic abnormalities are related to certain genes in the horse’s DNA. Congenital abnormalities can range from contracted tendons and angular limb deformities to heart ventricle defects and intestinal atresia (blockage). Your veterinarian can treat some less-severe congenital defects, but more severe cases usually end up in foal death.

Both genetic and congenital abnormalities can affect multiple body systems. Some examples of genetic disorders are HYPP in Quarter Horses, Warmblood Fragile Foal system, and Polysaccharide Storage Myopathy in multiple breeds. A wide range of genetic tests are available to help breeders avoid crossing carriers with carriers; some breed associations insist on such testing before horses can be registered. Researching different disorders before selecting a breeding pair is important to prevent any problems. There is no cure for genetic conditions, although some can be successfully managed, allowing horses to live healthy lives.

Working closely with your veterinarian will allow for the best chance of a healthy outcome for both your mare and her foal.