Pregnancy Loss (Miscarriage) in Horses

By PetMD Editorial on Nov. 14, 2008

Abortion in Mares

It is not uncommon for horses to experience spontaneous abortions (miscarriages). A variety of medical reasons can cause this reaction, many of which depend on the gestational stage of the horse. In mares, abortion is defined as the failure of the fetus before it reaches the 300-day gestation period; anything after that period is considered to be an early delivery of the foal.

The normal gestation period for a healthy mare is 340 days. Any birth of a live or dead foal after 200 days is considered an early parturition.


  • Formation of milk in the mammary gland
  • Growth of the mammary gland
  • Vaginal discharge and bleeding


Infections that are either bacterial, fungal or viral in nature can all cause spontaneous abortion in mares. Bacterial or fungal infections can occur in the female's uterus or the placenta, while the Equine Herpes Virus Type 1 (EHV-1) is transmitted to the animal in a variety of ways. Some of the other causes of spontaneous abortion in horses include:

  • Gene mutations
  • Twisting of the umbilical cord
  • Mare Reproductive Loss Syndrome (MRLS)
  • Lack of sufficient nutrients to support the fetus (especially seen in the case of twins)


The cause of the spontaneous abortion may sometimes remain unclear, but there are usually underlying medical reasons. The fetus can be examined via autopsy at a laboratory. The mare should also be swabbed and samples sent to the laboratory for diagnosis in order to determine whether the problem was with the fetus or with the mare.


The course of treatment will ultimately depend on the cause of the abortion. For example, if the abortion was caused by infection, the veterinarian will prescribe medication to eliminate said infection. The gestational period in which the abortion occurred in the mare will also be a factor in the type of treatment the animal receives.


Living and Management

A healthy mare is critical to a successful pregnancy and a healthy foal. While you may not have much control over the proper development of the fetus, caring properly for an expecting mare is half the battle. Proper feeding with proper supplementation is always a good idea for a healthy mare and foal.


Mares that are normally less healthy than other horses should not be allowed to become pregnant; likewise with older mares. It may also be a smart idea to keep a pregnant horse away from other horses in the farm during the period of gestation.

You know your horse better than anyone. Pay close attention to their health, their attitude, and their comfort. If you feel any doubt about the quality of their health, seek the advice of your veterinarian as soon as possible.

It is also important to vaccinate against EHV-1, which can cause spontaneous abortion in mares.

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