A comparison of cervical length measurement techniques for the prediction of spontaneous preterm birth

Objective: To evaluate the clinical utility of a novel means of assessing the cervix by measuring the angle of the curvature and to evaluate the performance of this technique as well as two other commonly used techniques of cervical length assessment in predicting spontaneous preterm birth (SPTB). Methods: This was a retrospective cohort analysis of singleton gestations with a history of SPTB. Transvaginal ultrasound images of cervical length obtained between 20 and 23 6/7 weeks were re-measured using three techniques: (1) straight linear distance between the internal and external os, (2) sum of two contiguous linear segments tracing the internal to the external os, and (3) measurement of the angle of the curve within the cervix using an electronic protractor. A short cervical length was defined as ≤25 mm. Results: A total of 181 women were included. The relative risk (RR) for SPTB by cervical angle ≤160° was 1.2 (95% CI 0.7–2.0) and the ROC curve revealed an area under the curve of 0.54 (95% CI 0.44–0.63). The RR for SPTB by short cervical length measured by the straight technique was 2.3 (95% CI 1.3–4.0) and by the segmental technique 2.1 (95% CI 1.2–3.8). There was a 99.4% agreement between the two techniques with an intraclass Kappa coefficient of 0.96. Conclusions: In women with a history of SPTB, cervical angle measurement does not correlate with the risk of SPTB. Cervical length measured via straight and segmental techniques had excellent agreement in identifying short cervix, and both identified a short cervix predictive of SPTB.