ObjectiveTo determine the normal concentrations of maternal serum interleukin-6 during the second and third trimesters of pregnancy and the different stages of term and preterm labor, and to examine the clinical usefulness of measuring this cytokine in the serum of women in preterm labor to diagnose asymptomatic intrauterine infections.MethodsMaternal serum interleukin-6 concentrations were measured cross-sectionally in 315 gravidas in their second and third trimesters and during term and preterm labor. Placentas from women who delivered preterm were examined for histologic chorioamnionitis.ResultsAt term, women in labor had significantly elevated median maternal serum interleukin-6 concentrations compared with those at term not in labor (4.7 pg/mL versus 2.2 pg/mL, P < .001). Women admitted in preterm labor who delivered had significantly higher median interleukin-6 concentrations than did those in preterm labor who responded to tocolysis (9.3 pg/mL versus 1.9 pg/mL, P < .001). Women in preterm labor who delivered preterm with evidence of chorioamnionitis had significantly higher serum concentrations of interleukin-6 than did those in preterm labor who delivered in the absence of chorioamnionitis (15.9 pg/mL versus 4.6 pg/mL, P = .006).ConclusionCompared with antepartum gravidas, those in term or preterm labor had significantly higher concentrations of maternal serum interleukin-6 concentrations; extremely elevated levels were found in patients whose preterm labor was associated with intrauterine infection.