Our aim was to compare the Bishop score, FNf assays, and the sonographic measurement of cervical length, in order to assess the best marker for time until spontaneous labor at term and risk of cesarean, especially for lack of progress of dilatation. This prospective study included 128 singleton vertex pregnancies with no clinical evidence of membrane rupture or regular contractions and a prenatal consultation between 39 weeks 4 days and 40 weeks 3 days. We successively assayed FNf, determine the Bishop scored, and measured cervical length by transvaginal ultrasound. The end points were the percentage of patients with a spontaneous onset of labor in the week following these tests and the type of delivery. The spontaneous onset of labor within a 7-day period was closely associated with a Bishop score > or = 6 and especially with a cervical length < or = 26 mm, but not with a positive FNf assay. On the other hand, vaginal delivery was significantly associated with the fibronectin assay but not with either the Bishop score or cervical length. Ultrasound measurement of cervical length and the fetal fibronectin assay provide different physiological data useful for different purposes: cervical length is more valuable than the Bishop score for predicting the onset of spontaneous labor within 7 days (when these assessments are performed close to term) and the FNf assay is very useful for evaluating the risk of cesarean.