On this occasion I want to comment on some structures that form in the uterus of pregnant mares and which are of vital importance for maintaining pregnancy in the first days.

Endometrial cups are identified as irregular, circular-appearing structures in the uterine horns of a pregnant mare. Their development begins approximately on day 25 of gestation, reaching their maximum size on day 70 and disappearing around day 130 of gestation.

Its function is basically the production of Equine Chorionic Gonadotropin (eCG), this in turn has the purpose of promoting the formation of supplementary corpora lutea and maintaining the primary CL, in order to produce sufficient levels of Progesterone (P4) to maintain the gestation until approximately days 150 to 200 of gestation, when the placenta would become the main source of P4. The peak of eCG occurs between days 55 and 65 of pregnancy, it also has an important role as an immunoregulator.

The days in which this hormone has its action and in which the supplementary CL are formed must be taken into account, since when a mare aborts after the formation of the endometrial cups it will take at least three months to cycle again while it is causes the degeneration of the cups and thus allowing the regression of the supplementary bodies, even if these mares are induced into estrus they may have anovulatory follicles.