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The goal of the current work was to establish histological changes in the endometrium, differences in serum concentrations of progesterone, 17β estradiol, insulin-like growth factor type 1, leptin and adiponectin of seas during early gestation compared to cyclic seas, and to verify associations between variables. During an empty period (cyclic mares) and a pregnant period. Radioimmunoassay, from ovulation to 13 days of estrous cycle and pregnancy, established hormones daily. In addition to quantifying the inflammatory cells in the endometrial tissue, endometrial biopsies were taken on days 7, 10 and 13 of the estrous cycle and pregnancy during which a histomorphological analysis of the endometrial glandular structure and luminal epithelium was performed. No variations were observed in the height of the luminal epithelium between the sampling days or between the groups within the histomorphological measurements; however, higher values were reported in pregnant seas than in cyclic seas at GE height, GD, GLdiameter and intraglandular secretions. Epithelium and glandular diameter decreased in both groups during the sampling days, while lumen and glandular secretions increased in the pregnant group during the days, not finding group differences. A greater number of eosinophils and lymphocytes were found in pregnant seas on day 7 than in cyclic seas, all cell types were decreased in pregnant seas during the sampling days. Of the serum hormones tested, no variations were observed between the study groups except for serum insulin, which appeared to be higher in the cyclical group than in the pregnant group; with higher concentrations in cyclical mares on days 8 and 12 than in pregnant ones. In conclusion, early pregnancy was associated with changes in endometrial and inflammatory cells in endometrial tissue. Changes in serum insulin were identified during early pregnancy. Only progesterone was associated with histomorphological changes in both classes of seas and with changes in inflammatory cells in pregnant seas, which in turn interrelated the glandular changes.