How is it used? Fetal fibronectin (fFN) is used to test pregnant women who are between 22 weeks and 35 weeks of pregnancy and are having. Fetal fibronectin (fFN) is a fibronectin protein produced by fetal cells. Key words: Fetal fibronectin, Spontaneous preterm birth, Cervical length. Our selection criteria were studies in asymptomatic or symptomatic pregnant women, cervicovaginal fetal fibronectin testing before 37 weeks' gestation, known.