Objective of the study: to study peculiarities of the inflammatory reactions development in placenta according to results of a general histological and immunohistochemical study of the prevalence and intensity of reactions caused by CD45 and tumor necrosis factor α (TNF-α) during childbirth at different gestational periods.Materials and methods. Morphological and immunohistochemical studies of various placental structures and CD45 and TNF-α expression level in 142 women with preterm delivery at different gestational periods were performed: group I – at 23–29 weeks, group II – at 30–34 weeks; group III – at 35–36 weeks + 6 days, group IV (control) – 40 women at 38–40 weeks.Results. Inflammatory infiltration and the most expressive CD45 expression prevailed in the decidual membrane, TNF-α expression was predominantly negative in group I. In group II, inflammatory changes were evidenced by collagenization and stromal fibrosis, a decrease in fetal vessels, fibroblast proliferation, an increase in intervillous fibrinoid, and CD45 spread was expressed in decreasing order of intensity in decidual membrane, in stroma of intermediate villi and in stem villi. In group III, foci of inflammatory infiltration spread from the decidual membrane to the intervillous space and villi stroma, vessels plethora of all types of villi, dystrophic changes, and fibrinoid volume increased. CD45 expression was more common and expressive than in the groups I and II, and TNF-α expression was weakly positive only in the decidual membrane. The control group determined the widespread expressive CD45 expression in all placental structures and positive moderate TNF-α expression.Conclusions. Preterm delivery and delivery on term occur when there is an activation of inflammatory reactions in placenta. Inflammation in the placental structures is a necessary condition for development of the mother-fetal interphase, however, the features of local immune response prove the need to determine genetic mechanisms of the preterm labor.