Objective Cervical cerclage is an important treatment used to prevent spontaneous preterm birth (sPTB) but it is not universally successful. Understanding the factors associated with cerclage treatment failure may lead to improved patient selection and better patient outcomes. The objective of this study was to investigate the value of pre-and post-cerclage cervical length (CL) in predicting sPTB < 34 weeks.

Methods This was a retrospective cohort study conducted in four preterm birth prevention clinics in the UK. We included 331 women who had undergone cervical cerclage between January 2008 and March 2021 and analyzed their pre- and post-cerclage CL, as measured by transvaginal ultrasound scan. The primary outcome was sPTB < 34 weeks’ gestation, assessed using multivariable logistic regression modelling (variables were pre- and post-cerclage CL, gestational age at cerclage and direction of CL change), and the generation of receiver-operating-characteristic (ROC) curves. Differences in sPTB for underlying risk factors (race, history of smoking, previous cervical surgery or pregnancy history risk factors, including MTL or sPTB) were assessed using Fisher’s exact test.

Results Both pre- and post-cerclage CL were independently discriminative of sPTB < 34 weeks’ gestation, with areas under the ROC curve of 0.635 and 0.677, respectively, and were modest contributors to sPTB prediction based on multivariable logistic regression modelling (odds ratio (OR), 0.964 (95% CI, 0.936–0.994); p = 0.018 and OR, 0.940 (95% CI, 0.910–0.970); p< 0.001, respectively). There were no significant differences in the rate of sPTB < 34 weeks’ gestation according to race, history of smoking, previous cervical surgery or pregnancy history risk factors, including previous MTL or sPTB.

Conclusions Post-cerclage CL is the predominant predictor of sPTB < 34 weeks’ gestation. Underlying sPTB risk factors (history of cervical surgery and pregnancy history) may influence pre-cerclage CL and the direction of CL change following cerclage, but once these are adjusted for, they do not influence the risk of sPTB < 34 weeks’ gestation.