Subclinical endometritis is more common in older mares (16–23 years old), with increased secretion of arachidonic acid metabolites such as prostaglandin E2, prostaglandin-1α, and leukotriene-C4, which deteriorate the immunoendocrine balance [38] and reduce the expression of progesterone receptors [39], resulting in significantly reduced fertility. This evidence concerns the gene PGR and endometritis.