Subclinical and clinical chorioamnionitis, fetal vasculitis, and risk for preterm birth: A cohort study

To evaluate the association between subclinical and clinical chorioamnionitis and risk of preterm birth (PTB). Demographic and clinical characteristics were abstracted from medical records and placental examinations performed (N = 1371 pregnancies including spontaneous and medically-indicated PTBs). Pregnancies were classified as having clinical chorioamnionitis (with or without histologic chorioamnionitis), subclinical chorioamnionitis (histologic, but not clinical, chorioamnionitis), or no chorioamnionitis; pregnancies with histologic chorioamnionitis were further evaluated for fetal vasculitis. Relative risks for PTB, early and late PTB, and PTB ± premature rupture of membranes (PROM) were adjusted for maternal characteristics. Clinical (4.3%) and subclinical (24.5%) chorioamnionitis were not associated with PTB overall. In pregnancies without clinical or subclinical chorioamnionitis, the risk of PTB with PROM and early PTB was 2.2% and 8.6%, respectively. In comparison, clinical chorioamnionitis was associated with an increased risk of PTB with PROM (aRR: 3.42 (95%CI: 1.07, 10.98), whereas subclinical chorioamnionitis was associated with increased risk of PTB with PROM (aRR: 3.92 (95% CI: 2.15, 7.12)) and early PTB (aRR: 1.77 (95% CI: 1.18, 2.64)). Histologic chorioamnionitis with fetal vasculitis was associated with increased risk of PTB with PROM (aRR: 7.44 (95% CI: 3.68, 15.05)) and early PTB (aRR: 2.94 (95% CI: 1.78, 4.87)), whereas histologic chorioamnionitis without fetal vasculitis was associated with increased risk of PTB with PROM only (aRR: 2.64, 95% CI: 1.27, 5.50). Subclinical chorioamnionitis and histologic chorioamnionitis with fetal vasculitis were associated with early PTB and PTB with PROM but not with PTB overall, likely due to inclusion of indicated PTBs. •Subclinical chorioamnionitis (CAM) affected one quarter of all pregnancies.•Subclinical CAM was not associated with preterm birth (PTB) overall.•Subclinical CAM was associated with increased risk of early PTB and PTB with PROM.•Fetal vasculitis was associated with increased risk of early PTB and PTB with PROM.