Did you think your mare was in foal but now that the time has come for her to foal, she is obviously not pregnant? Abortion in horses may result from a variety of causes. Infectious agents, such as bacteria, viruses or fungi, may attack the fetus or its membranes, resulting in fetal death and expulsion. Other factors attributable to the mare, fetus or external forces may also cause loss of the embryo or fetus. These factors include twinning, hormonal deficiencies, congenital anomalies, ergot alkaloid toxicity or ingestion of tent caterpillar setae. Some of the more common causes are discussed in this article by Dr. Dan Kenny - Diplomate/A.C.V.I.M./Ontario Veterinary College/University of Guelph.