Predictive Value of Initial Cervical Length for Subsequent Cervical Length Shortening in Women with a Prior Preterm Birth

Objective: The objective of this study was to estimate the predictive value of the initial cervical length (CL) for later CL shortening in women at high risk for recurrent spontaneous preterm birth (SPTB). Materials and

Methods: We conducted a secondary analysis of a multicenter randomized trial of high-risk women with CL screening scheduled at least every 2 weeks from 16(0/7) to 22(6/7) weeks. Primary outcome was CL < 25 mm or SPTB before follow-up CL; the risk of this outcome was analyzed by initial CL at 16(0/7) to 21(6/7) weeks using multivariable logistic regression and receiver operator curves.

Results: A total of 786 women were identified in the parent trial; 197 (25.0%) developed CL shortening < 23 weeks, 3 (0.4%) delivered within 2 weeks of initial CL measurement (< 24 weeks), and 586 (74.6%) maintained CL ≥ 25 mm. The covariate-adjusted receiver operator characteristics curve had moderate predictive value (area under the curve = 0.748) but did not reveal a clinically useful CL cutoff. Evaluation of selected CL cutoffs demonstrated that an initial CL ≥39 mm had the greatest negative predictive value (85.1%). No CL cutoff existed above which there was no CL shortening, although at ≥ 39 mm only two patients (1.1%) shortened within 2 weeks as compared with 32 (7.1%) patients whose initial CL was < 39 mm (p < 0.001).

Conclusion: In high-risk patients whose initial CL is ≥ 39 mm, CL shortening or delivery is uncommon in the following 2 weeks. Patients with initial CL above this threshold may require less frequent surveillance.