OBJECTIVE: Nitric oxide may be a factor in cervical ripening. We compared the nitric oxide metabolite levels in cervical fluid in women going beyond term and in women delivering spontaneously at term. METHODS: We studied a total of 208 women with singleton pregnancies: 108 women who went beyond term (294 days or longer), and 100 women who went spontaneously into labor at term. Cervical fluid samples, collected well before the initiation of labor, were assessed for nitric oxide metabolites using an assay with a detection limit of 3.8 μmol/L. RESULTS: Women going beyond term had detectable levels of nitric oxide metabolites in their cervical fluid (60%) less often (P = .001) than women delivering at term (87%). The nitric oxide metabolite concentration in cervical fluid in women going beyond term (median 23.5 μmol/L; 95%confidence interval less than 3.8, 31.8) was 4.5 times lower (P < .001) than that in women delivering at term (median 106.0 μmol/L; 95%confidence interval 81.8, 135.0). Such a difference (14.0 versus 106.0 μmol/L) also existed when only the 66 women going into spontaneous postterm labor were included in the comparison. Both nulliparous (median less than 3.8 μmol/L) and parous (median 31.3 μmol/L) women going beyond term had lower (P < .01) cervical fluid nitric oxide metabolite levels than nulliparous and parous women delivering at term (medians 76.1 and 101.3 μmol/L, respectively). In the postterm group, women with cervical fluid nitric oxide metabolite concentrations at or below the median failed more often (P < .001) to progress in labor and had longer (P = .02) duration of labor than those with cervical fluid nitric oxide metabolite concentrations above the median. CONCLUSION: Reduced cervical nitric oxide release may contribute to prolonged pregnancy. OBJECTIVE: Nitric oxide may be a factor in cervical ripening. We compared the nitric oxide metabolite levels in cervical fluid in women going beyond term and in women delivering spontaneously at term. METHODS: We studied a total of 208 women with singleton pregnancies: 108 women Who went beyond term (294 days or longer), and 100 women who went spontaneously into labor at term. Cervical fluid samples, collected well before the initiation of labor, were assessed for nitric oxide metabolites using an assay with a detection limit of 3.8 μmol / L. RESULTS: Women going beyond term had detectable levels of nitric oxide metabolites in their cervical fluid (60%) less often (P = .001) than women delivering at term (87%). The nitric oxide metabolite concentration in cervical fluid in women going beyond term (median 23.5 μmol / L; 95% confidence interval less than 3.8, 31.8) was 4.5 times lower (P <.001) than that at women delivering at term 81.8, 135.0) Both nulliparous (median less than 3.8 μmol / L) and parous (median 31.3 μmol / L) also showed when only the 66 women going into spontaneous postterm labor were included in the comparison. women going beyond term had lower (P <.01) cervical fluid nitric oxide metabolite levels than nulliparous and parous women delivering at term (medians 76.1 and 101.3 μmol / L, respectively). In the postterm group, women with cervical fluid nitric oxide metabolite concentrations at or below the median failed more often (P <.001) to progress in labor and had longer (P = .02) duration of labor than those with cervical fluid nitric oxide metabolite concentrations above the median. CONCLUSION: Reduced cervical nitric oxide release may contribute to prolonged pregnancy.