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.

Fertilization in the mare is a fairly complex process:

When the egg matures, the oocyte sheds from the follicle into the oviduct, Ball said. At the time of ovulation, the cumulus cells surrounding the egg undergo a massive expansion to form a sticky cell mass allowing the egg to adhere to the cilia (microscopic hairs) in the oviduct.

The egg then travels down into the fimbria, a fringe of tissue at the oviduct’s opening. A glandular structure called the corpus luteum is organized from cells producing the hormone progesterone, which is needed to establish and maintain pregnancy until the placenta comes into function at about 150 days of gestation.

Sperm are transported to the oviduct initiating fertilization prior to ovulation and, in most cases, are stored there for several days. The sperm remain viable for two or three days in the oviduct, while the egg is only viable for about six hours after ovulation