ObjectiveTo stratify the risk of spontaneous preterm delivery using cervical length (CL) and fetal fibronectin (fFN) in women with threatened preterm labor who remained pregnant after 7days. DesignProspective observational study. SettingNationwide cohort of women with threatened preterm labor from the Netherlands. PopulationWomen with threatened preterm labor between 24 and 34weeks with a valid CL and fFN measurement and remaining pregnant 7days after admission. MethodsKaplan-Meier and Cox proportional hazards models were used to estimate cumulative percentages and hazard ratios (HR) for spontaneous delivery. Main outcome measuresSpontaneous delivery between 7 and 14days after initial presentation and spontaneous preterm delivery before 34weeks. ResultsThe risk of delivery between 7 and 14days was significantly increased for women with a CL<15mm or a CL 15 to <30mm and a positive fFN, compared with women with a CL 30mm: HR 22.3 [95% confidence interval (CI) 2.6-191] and 14 (95% CI 1.8-118), respectively. For spontaneous preterm delivery before 34weeks the risk was increased for women with a CL<15mm [HR 6.3 (95% CI 2.6-15)] or with a CL 15 to <30mm with either positive fFN [HR 3.6 (95% CI 1.5-8.7)] or negative fFN [HR 3.0 (95% CI 1.2-7.1)] compared with women with a CL30mm. ConclusionsIn women remaining pregnant 7days after threatened preterm labor, CL and fFN results can be used in risk stratification for spontaneous delivery.