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Although a history of preterm birth raises risk for recurrence in subsequent pregnancies, most early deliveries occur in first-time mothers without known risk factors. Both transvaginal ultrasound to measure cervical length (CL) and quantitative assessment of fetal fibronectin (fFN) have been proposed as tools to detect excess risk for preterm birth, and use of CL measurement is widespread; however, neither screening strategy has proven accurate. In a prospective observational cohort study involving 9410 nulliparous women with singleton pregnancies at eight U.S. sites, investigators evaluated the characteristics of CL and fFN as predictive indices. Samples of fFN were collected during each trimester; CL was measured at 16 to 23 and 22 to 31…