Background: The formation of a solid uterine scar depends on how adequate reparative process is in the uterine wall. The majority of researchers observe a decrease in the number of maternity patients with a uterine scar having to undergo caesarean section. This requires necessity to improve diagnostics of the condition of the postoperative uterine scar after the caesarean section, thus giving more opportunities for natural vaginal delivery. The aim of the study: To provide the assessment of immunohistochemical characteristics of post-surgical changes in the myometrium in order to identify a high risk group of maternity patients, who do not have a solid uterine scar and thus risk developing hypotonic bleeding during the next pregnancy. Materials and methods: The research involved 47 pregnant patients undergoing morphological and immunohistochemical examination of the myometrium of the lower uterine segment. Out of these, 27 studied samples were taken from the lower uterine segment, affected by the scar after the earlier caesarean section. The other 20 samples have unaffected myometrium taken in the section area during the first caesarean section. Results: The conducted morphological research demonstrates the decrease in the function of the myometrium of the lower uterine segment after the earlier caesarean section. It is accounted for the fact of its shrinking in relation to the amount of tissue and the level of the expression of nonstriated actine in smooth myocites. What is equally important is the presence of the fibrous component of extra-cellular matrix of connective tissues between preserved clusters of smooth myocites, which combines with local loss of reticular fibers contacting their basic membrane. As a result, the formation of the uterine scar goes along with interruption in the functional activity of smooth myocites, which becomes the basis for the decrease in their shrinking ability as a whole layer. Simultaneously, we could observe an increase in the number of mast cells and their secretory capacity. The intact myometrium demonstrates doubling of non-degranulated mast cells, whereas the uterine scar shows that the number of degranulated mast cells increases as much as 2.4. Moreover, mast cells in the scar become bigger in size and contain more tryptase, which proves their active role in pathogenesis of the formation of the cicatrical tissue. Conclusion: The increase in the number of mast cells shows that they are involved in pathogenesis of the formation of the cicatrical tissue, which leads to the decrease in the normal shrinking activity of myometrium.