Introduction Fetal fibronectin (fFN) testing is an established predictor for spontaneous preterm birth (sPTB). A novel bedside fFN analyser allows quantification of fFN (qfFN) concentrations. It is commonly thought that cerclage insertion increases ‘false positive’ rates, however neither the effect of cerclage insertion on qfFN levels, nor the relationship between pre- and post-cerclage qfFN concentration and sPTB have been described.

Methods Prospective predefined analysis of a cohort of asymptomatic women at high risk of sPTB. Women in whom an ultrasound-indicated cerclage had been inserted (cervical length <25 mm, 18⁺⁰–25⁺⁰ weeks) with serial qfFN results were included.

Results 285 qfFN results from 51 women were available. sPTB rate <30 weeks was 33% (17/51). There was no significant difference between mean pre-cerclage qfFN concentration (94.5 ng/ml, 95% CI 56.9–156.9) and mean post-cerclage (up to two weeks) concentrations (117.2 ng/ml, 82.8–166.0). Whilst the absolute post-cerclage qfFN concentration predicted sPTB <30 weeks with an area under the receiver operating characteristic (ROC) curve of 0.72 (0.58–0.87), the ratio between pre- and post-cerclage levels had a superior ROC area of 0.85 (0.73–0.96). A 75% increase in fFN at 2–4 weeks post-cerclage had a 92% sensitivity for sPTB <30 weeks’ gestation. >50% or greater decrease in qfFN was associated with 100% sensitivity and 84% specificity for delivery >30 weeks’ gestation.

Conclusion Insertion of emergency cerclage does not appear to significantly affect qfFN levels. The predictive power of post-cerclage qfFN concentrations for sPTB <30 weeks’ gestation was enhanced by consideration of the pre- and post-cerclage qfFN ratios.