BACKGROUND: Congenital uterine anomalies (CUA) are associated with late miscarriage and spontaneous preterm birth (sPTB). OBJECTIVES: Our aim was to 1) determine the rate of sPTB in each type of CUA and 2) assess the performance of quantitative fetal fibronectin (qfFN) and transvaginal cervical length (CL) measurement by ultrasound in asymptomatic women with CUA for the prediction of sPTB at <34 and <37 weeks of gestation. STUDY DESIGN: This was a retrospective cohort of women with CUA asymptomatic for sPTB, from four UK tertiary referral centres (2001-2016). CUAs were categorised into fusion (unicornuate, didelphic and bicornuate uteri) or resorption defects (septate, with or without resection and arcuate uteri), based on pre-pregnancy diagnosis. All women underwent serial transvaginal ultrasound CL assessment in the second trimester (16 to 24 weeks' gestation); a subgroup underwent qfFN testing from 18 weeks' gestation. We investigated the relationship between CUA and predictive test performance for sPTB before 34 and 37 weeks' gestation. RESULTS: Three hundred and nineteen women were identified as having CUA within our high-risk population. 7% (23/319) delivered spontaneously <34 weeks, and 18% (56/319) <37 weeks' gestation. Rates of sPTB by type were: 26% (7/27) for unicornuate, 21% (7/34) for didelphic, 16% (31/189) for bicornuate, 13% (7/56) for septate and 31% (4/13) for arcuate. 80% (45/56) of women who had sPTB <37 weeks did not develop a short CL (<25 mm) during the surveillance period (16-24 weeks). The diagnostic accuracy of short CL had low sensitivity (20.3) for predicting sPTB <34 weeks. Cervical Length had ROC AUC of 0.56 (95% CI 0.48 to 0.64) and 0.59 (95% CI 0.55 to 0.64) for prediction of sPTB <34 and 37 weeks' respectively. The AUC for CL to predict sPTB <34 weeks was 0.48 for fusion defects (95% CI 0.39 to 0.57) but 0.78 (95% CI 0.66 to 0.91) for women with resorption defects. Overall quantitative fetal fibronectin had a AUC of 0.63 (95% CI 0.49 to 0.77) and 0.58 (95% CI 0.49 to 0.68) for prediction of sPTB <34 and 37 weeks, respectively. AUC for prediction of sPTB <37 weeks with qfFN for fusion defects was 0.52 (95% CI 0.41 to 0.63), but 0.79 (0.63 to 0.95) for women with resorption defects. Results were similar when women with intervention were excluded. CONCLUSION: Commonly used markers CL and qfFN have utility in prediction of sPTB in resorption congenital uterine defects but not in fusion defects. This is contrary to other high-risk populations. These findings need to be accounted for when planning antenatal care and have potential implications for predictive tests used in sPTB surveillance and intervention.