According to Barry Ball, DVM, PhD, Dipl. ACT, the Albert G. Clay Endowed Chair in equine reproduction at the University of Kentucky’s Gluck Equine Research Center, there are a variety of potential causes for pregnancy loss in mares.

Early pregnancy loss is widely associated with a mare’s age, followed by embryo, oviductal, uterine, or endocrine factors, Ball said. Older or multiparous (having foaled two or more times) mares have an increased risk of developing abnormal numbers of chromosomes (aneuploidy) presumably due to a prolonged follicular growth.

"As the mare ages, so do her oocytes," Ball said. Thus, the risk of uterine impairment might result from the mare’s normal aging processes