In most mammalian species, progestins have a major function in maintaining pregnancy. In humans, the physiologic initiation of parturition bears similarities with inflammatory processes and anti-inflammatory effects of progestins have been suggested to postpone birth until term. To examine if comparable effects exist in the horse, mares were treated with the synthetic progestin altrenogest from day 280 of gestation until parturition (N = 5) or were left untreated as controls (N = 7). Tissue from the amnion (AMN), allantochorion (AC), and endometrium (EM) was collected at foaling and mRNA expression of interleukin (IL)-6 and -8, cyclooxygenase 2 (COX2), estrogen receptor (ER) α, progesterone receptor, and oxytocin receptor (OTR) was analyzed. Leukocytes, steroid receptors, COX2, and OTR were also investigated by histology and immunohistochemistry. Expression of mRNA for IL-6 was higher in AMN and EM versus AC (P < 0.01). Expression of IL-8 was higher in AMN than AC and EM (P < 0.001). Steroid receptors and OTR were highly expressed in EM but not in AMN and AC (P < 0.001). Expression of COX2 was most pronounced in AC whereas IL expression was not upregulated in AC. No differences in mRNA expression existed between altrenogest-treated and control animals. Endometrial polymorphonuclear leukocytes were increased in altrenogest-treated mares. Epithelial cells of all tissues, except AC chorionic villi stained progesterone receptor-positive. Staining for ER was more pronounced in the amnion facing epithelium of the AC in altrenogest-treated versus control animals (P < 0.01). In conclusion, COX2 is highly expressed in the AC. The fetal membranes thus might play a role in the onset of labor in the horse. Altrenogest did not affect gene expression in the AMN, AC, and EM but had localized effects on inflammatory cells and ER expression. No anti-inflammatory effects of altrenogest in healthy, late pregnant pony mares could be detected.