Objective The overtly healthy, nonpregnant uterus harbors bacteria, Mycoplasma and Ureaplasma . The extent of colonization remains elusive, as are relationships between isolated microorganisms, preterm labor and fetal inflammation. Study Design Biopsy specimens of chorion parenchyma from 1083 placentas delivered before the beginning of the 28th week of gestation were cultured, and the placentas were examined histologically. The frequencies of individual microorganisms and groups of microorganisms were evaluated in strata of processes leading to preterm delivery, routes of delivery, gestational age, and placenta morphology. Results Placentas delivered by cesarean section with preeclampsia had the lowest bacterial recovery rate (25%). Preterm labor had the highest rates, which decreased with increasing gestational age from 79% at 23 weeks to 43% at 27 weeks. The presence of microorganisms in placenta parenchyma was associated with the presence of neutrophils in the fetal stem vessels of the chorion or in the vessels of the umbilical cord. Conclusion The high rate of colonization appears to coincide with phenomena associated with preterm delivery and gestational age. The presence of microorganisms within placenta parenchyma is biologically important.