Objective: To relate vaginal fetal fibronectin levels in women with symptoms of preterm labor to subsequent spontaneous preterm birth. Methods: Quantitative fetal fibronectin values were calculated from women who participated in two prospective multicenter trials relating fetal fibronectin to subsequent spontaneous preterm birth. The study populations consisted of women who presented with symptoms of preterm labor between 24 0/7 and 34 6/7 weeks, a singleton pregnancy, intact membranes, no prior tocolysis, and cervical dilation less than 3 cm. Results: The characteristics of the two study populations were similar. In both populations, the rates of delivery within 7, 14, and 21 days after sampling were clustered into three distinct fetal fibronectin groups (less than 40, 40-100, and 100 ng/mL or more). As fetal fibronectin values increased, the risk of subsequent spontaneous preterm birth also increased. Delivery within 7 days of sampling was 0.4%, 3.3%, and 18.2% (trial A) and 1.4%, 8.0%, 30.0% (trial B) as the fetal fibronectin levels increased from less than 40 ng/mL, to 40-100 ng/mL, and to at least 100 ng/mL, respectively. Conclusion: In women with symptoms of preterm labor, an increase in fetal fibronectin from under 40 ng/mL, to 40-100 ng/mL, to at least 100 ng/mL was associated with a progressive increase in the risk of subsequent spontaneous preterm birth. The use of a single fetal fibronectin cutoff of 50 ng/mL for defining a positive test in women with symptoms of preterm labor should be reevaluated. © 2001 by The American College of Obstetricians and Gynecologists.