HYPOTHESIS/AIMS OF STUDY: Infection of the amniotic cavity and placenta is one of the leading causes of adverse pregnancy outcomes. In the majority of cases, intra-amniotic infection is associated with the normal microbiota of the lower urogenital tract. The aim of the study was to explore the relationships between the placental inflammatory changes, vaginal microbiota and labor course. STUDY DESIGN, MATERIALS AND METHODS: We examined 124 women at 37-41 weeks of gestation. The vaginal discharge at admission was taken for microbiological evaluation, with the delivered placenta sent for histological examination. RESULTS: In 17.7% of cases, histological examination of the placenta revealed inflammatory changes. A statistically significant correlation was noted between the placental inflammatory changes and Staphylocossus spp. presence in the vaginal discharge at admission (р = 0.0004). The placental inflammatory changes were associated with the membrane rupture to delivery interval more than 6 hours (р = 0.01) and the labor duration more than 7 hours (р = 0.0004). Prelabor rupture of membranes did not significantly affect the placental inflammatory changes (p = 1.0). CONCLUSION: Predisposing factors for the development of ascending bacterial infection of the placenta are an abnormal vaginal microbiota with the presence of opportunistic bacteria before delivery, a long membrane rupture to delivery interval, and a prolonged labor.