Senescence in placental cells impacts physiological functions and contributes to pathology. Therefore, we examined biochemical markers of cellular senescence—p38, P-p38, p21, and p53—during pregnancy, at parturition, and in cases of retained fetal membranes. Placentomes were collected from pregnant cows (2nd, 4th, and 5th months of gestation) and parturient cows undergoing cesarean section, categorized as NR or RFM. Samples were separated into maternal and fetal parts and analyzed via WB and ELISA. WB confirmed protein presence, while ELISA showed a significant increase in the concentrations of both p38 and P-p38 in the fetal part in the 5th month of gestation as compared to earlier months. No significant differences were observed in the maternal part across pregnancy and parturition. These findings suggest p38 and P-p38 may be important molecules regulating the normal development of the bovine placenta during middle pregnancy. Further studies are needed to elucidate the mechanisms of their action.