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Objective: To determine whether fetal fibronectin is a sensitive test for the detection of amniotic fluid (AF) in women with rupture of the membranes., Methods: In a multicenter clinical trial, pooling, ferning, and nitrazine tests were compared with fetal fibronectin in 339 women at term (study group) with a clinical history of rupture of the membranes and in 67 women at term receiving routine prenatal care (controls). Ruptured membranes was diagnosed if any two of the standard tests were positive. Fetal fibronectin in the cervicovaginal secretions was determined using a qualitative enzyme-linked immunosorbent assay test. Fetal fibronectin was considered positive at 50 ng/mL., Results: The sensitivity of fetal fibronectin in the women with ruptured membranes was 98.2%. No controls had ruptured membranes based on standard test results, but 13 women had fetal fibronectin present. The mean (+/- standard error) interval between sampling and delivery was significantly less in the women positive for fetal fibronectin (169.3 +/- 45.8 hours) than in those with no detectable fetal fibronectin (333.4 +/- 29.0 hours)., Conclusion: Fetal fibronectin is a sensitive test for detection of AF in the vagina and compares favorably to standard tests. Its low specificity suggests that the assay may detect an alteration in membrane integrity. In addition, in patients without rupture of the membranes, the interval between sampling and delivery is significantly shorter if fetal fibronectin is present. We speculate that the presence of fetal fibronectin in cervicovaginal secretions may be a marker for impending labor in gravidas without gross rupture of the membranes.