BACKGROUND Histologic chorio-deciduitis and chorio-deciduo-amnionitis (amnionitis) in extra-placental membranes are known to represent the early and advanced stages of ascending intra-uterine infection. However, there are no data in humans about the time required for chorio-deciduitis to develop and for chorio-deciduitis without amnionitis to progress to chorio-deciduitis with amnionitis, and the effect of prolongation of pregnancy on the development of chorio-deciduitis and amnionitis in patients with preterm labor and intact membranes (PTL) and preterm premature rupture of membranes (preterm-PROM). We examined these issues in this study. METHODS The study population consisted of 289 women who delivered preterm (133 cases with PTL, and 156 cases with preterm-PROM) and who had sterile amniotic fluid (AF) defined as a negative AF culture and the absence of inflammation as evidenced by a matrix metalloproteinase-8 (MMP-8) level <23 ng/ml. We examined the association between amniocentesis-to-delivery interval and inflammatory status in the extra-placental membranes (i.e., inflammation-free extra-placental membranes, choroi-deciduitis only, and chorio-deciduitis with amnionitis) in patients with PTL and preterm-PROM. RESULTS Amniocentesis-to-delivery interval was longer in cases of chorio-deciduitis with amnionitis than in cases of chorio-deciduitis only in both PTL (median [interquartile-range (IQR)]; 645.4 [319.5] vs. 113.9 [526.9] hours; P = 0.005) and preterm-PROM (131.3 [135.4] vs. 95.2 [140.5] hours; P<0.05). Amniocentesis-to-delivery interval was an independent predictor of the development of both chorio-deciduitis and amnionitis after correction for confounding variables such as gestational age at delivery in the setting of PTL, but not preterm-PROM. CONCLUSIONS These data confirm for the first time that, in cases of both PTL and preterm-PROM with sterile AF, more time is required to develop chorio-deciduitis with amnionitis than chorio-deciduitis alone in extra-placental membranes. Moreover, prolongation of pregnancy is an independent predictor of the development of both chorio-deciduitis and amnionitis in cases of PTL with sterile AF.