Preterm birth is the leading cause of perinatal morbidity and mortality. Fetal fibronectin (fFN), a glycoprotein in the extracellular matrix of the amniotic membranes, is the most powerful biomarker for predicting the risk of preterm birth. Biosensors using the surface plasmon resonance (SPR) response are potentially useful in quantitatively measuring molecules. We established a standard calibration curve of SPR intensity against fFN concentration and used the SPR-based biosensor to detect fFN concentrations in the cervicovaginal secretions of pregnant women between 22 and 34 weeks of gestation. The calibration curve extends from 0.5 ng/mL to 100 ng/mL with an excellent correlation (R2
= 0.985) based on standard fFN samples. A cutoff value of 50 ng/mL fFN concentration in commercial ELISA kits corresponds to a relative intensity of 17 arbitrary units (a.u.) in SPR. Thirty-two pregnant women were analyzed in our study. In 11 women, the SPR relative intensity was greater than or equal to 17 a.u., and in 21 women, the SPR relative intensity was less than 17 a.u. There were significant differences between the two groups in regular uterine contractions (p
= 0.040), hospitalization for tocolysis (p
= 0.049), and delivery weeks (p
= 0.043). Our prospective study concluded that SPR-based biosensors can quantitatively measure fFN concentrations. These results reveal the potential utility of SPR-based biosensors in predicting the risk of preterm birth.