The Preterm Prediction Study: Quantitative fetal fibronectin values and the prediction of spontaneous preterm birth

OBJECTIVE: A cervicovaginal fetal fibronectin value of greater than or equa l to 50 ng/mL has been used to define women at risk of having a preterm bir th. We evaluated the relationship between quantitative fetal fibronectin va lues and spontaneous preterm birth. STUDY DESIGN: Cervical and vaginal specimens for fetal fibronectin were obt ained at 24, 26, 28, and 30 weeks' gestation from 2926 women. Quantitative fetal fibronectin values were calculated by using absorbances determined by enzyme-linked immunosorbent assay. The highest fetal fibronectin value (ce rvical or vaginal) for each woman at each visit was evaluated in relation t o spontaneous preterm birth at <35 weeks' gestation. Receiver operating cha racteristic curves were constructed to determine the optimal cutoff point f or fetal fibronectin values to predict spontaneous preterm birth at <35 wee ks' gestation and within 4 weeks of testing. RESULTS: The risk of spontaneous preterm birth increased as a function of i ncreasing fetal fibronectin values from approximately 20 to 300 ng/mL. Feta l fibronectin values greater than or equal to 300 ng/mL were not associated with a fur ther increase in spontaneous preterm birth. Examination of the receiver operating characteristic curve indicates that the optimal cutoff p oint for a positive fetal fibronectin test result at 24 to 30 weeks' gestat ion to predict spontaneous preterm birth at <35 weeks is between 45 and 60 ng/mL. CONCLUSION: Increasing levels of cervicovaginal fetal fibronectin up to 300 ng/mL are associated with an increasing risk of spontaneous preterm birth. Nevertheless, at 24 to 30 weeks, the value currently used, 50 ng of fetal fibronectin per milliliter appears to be a reasonable cutoff point for pred icting spontaneous: preterm birth at <35 weeks' gestation.