Pregnancy Loss (Miscarriage) in Horses

Cecilia de Cardenas
Nov 14, 2008
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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.

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