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Objective To investigate the causes and mechanism of preterm birth through analysis of the relationship between histological chorioamnionitis (HCA) in placental tissue and preterm birth.Methods Totally,327 preterm birth cases with report of placental pathologic examination were retrospectively collected from those women who delivered from December 1,2009 to December 1,2012 in Peking University First Hospital.According to the etiology of preterm birth,three groups were assigned:iatrogenic group (n=106),spontaneous contraction group (n=56) and premature rupture of membranes (PROM) group (n=165).According to the gestational age at delivery,three subgroups were further divided:early-preterm group (≥ 28-＜ 32 weeks),mid-preterm group(≥ 32-＜ 34 weeks) and late-preterm group (≥ 34-＜ 37 weeks).HCA was confirmed when ≥ 5 neutrophil infiltration identified on the chorionic plate and amniotic membrane under high power light microscope after HE staining.The relationship between HCA and the different types of preterm birth and the different delivery gestational age were analyzed.Besides,the consistency between clinical chorioamnionitis and HCA was also analyzed.Chi-square test was applied for statistics.Results The incidence of HCA in the spontaneous contraction group was significantly higher than in the iatrogenic group and PROM group [66.1％ (37/56) vs 25.5％ (27/106) and 33.3％ (55/165),x2=25.27 and 18.44,both P ＜ 0.01],but no significant difference was found between the latter two groups (P ＞ 0.05).Among the three subgroups,the early-,midand late-preterm subgroup,the incidence of HCA in the iatrogenic group was 24.0％ (6/25),33.3％ (8/24) and 22.8％ (13/57) (P ＞ 0.05),and 13/17,5/7 and 59.4％ (19/32) in the spontaneous contraction group (P ＞ 0.05).However,significantly higher incidence of HCA was shown in the early-preterm subgroup than in the mid-and late-preterm subgroup [70.0％ (20/29) vs 41.2％ (14/34) and 20.6％ (21/102),X2=4.87 and 24.58,both P ＜ 0.05] in the PROM group.Among the subjects in PROM group,those with the latency ≥ 72 h after the rupture of membranes had a higher incidence of HCA than those with the latency less than 72 h [68.6％ (24/35) vs 23.8％ (31/130),x2=24.82,P ＜ 0.01].For all 327 cases in this study,the incidence of clinical chorioamnionitis was 15.9％ (52/327),among which 31 cases [59.6％ (31/52)] were diagnosed as HCA.Conclusions The occurrence of HCA is closely associated with spontaneous contraction preterm.Some iatrogenic preterm birth might cause HCA.The earlier the preterm birth and the longer the latency after PROM,the higher the incidence of HCA.Differential diagnosis is necessary as the inconsistency between clinical chorioamnionitis and HCA.