Post-translational modifications (phosphorylation and glutathionylation) not only assure protein diversity but are also responsible for the controlling of the biological activity of selected proteins in health and disease. The aim of the study was to monitor the profile of changes in molecular weight of proteins undergoing selected post-translational modifications by measurement of the intensity of phosphorylation and glutathionylation within the pregnancy course and parturition in cows with and without the retention of foetal membranes. The intensity of selected post-translational modifications was measured in bovine placental tissues collected during pregnancy (2nd, 4th, 5th, and 7th month, n = 4 per month) and parturition (not-retained foetal membranes (NRFM, n = 6) and retained foetal membranes (RFM, n = 6). Placental tissues were homogenised and used for the Phosphoprotein Phosphate Estimation Assay Kit and Western blotting analyses with adequate antibodies. The content of phosphorylated proteins was significantly higher (p < 0.05) in the 2nd month as compared to other months, both in the maternal and foetal parts of the placenta. Moreover, no significant differences were observed between NRFM and RFM samples. The results of Western blotting showed the shift in molecular weight and indirect content of phosphorylated selected amino acids. Further research on the role of post-translational modifications in pregnancy and parturition may give new insight into their biochemical regulation.