Does the rate of preterm birth change when cervical length isgreater than 25 mm?

Abstract

Objective: To evaluate if the prevalence of preterm birth (PTB) diminishes as the cervix becomes longer than 25 mm.

Design: Retrospective cohort Participants/Materials, Setting, Methods: Transvaginal cervical length (CL) was measured in 3,376 singleton pregnancies without fetal anomalies between 17 and 24 weeks of gestation. Patients were classified as low-risk (LR, n=2,863; 84.8%) or high-risk (HR, n=513; 15.2%) based on previous history of PTB. CL measurements were stratified into the following groups: ≤15 mm, 15.1-25.0 mm, and thereafter in 5 mm increments up to 55 mm. Spontaneous PTB was defined as delivery <37 weeks and ≤34 weeks, not attributed to maternal or fetal indications (e.g., preeclampsia, fetal growth restriction). Prevalence of PTB was analyzed across CL categories and between risk groups.

Results: The overall prevalence of PTB <37 weeks was 21.7% (733/3,376), and of PTB ≤34 weeks was 5.5% (n=188/3,376). In LR patients, the prevalence of PTB <37 weeks was higher at CL ≤25 mm, and did not change when the cervix was >25 mm. In LR patients, the prevalence of PTB ≤34 weeks was higher at 25.1-30.0 mm, as compared to patients with CL >30 mm. Thereafter, the prevalence of PTB ≤34 weeks did not change. In HR pregnancies, the prevalence of PTB <37 weeks did not change at CL >15 mm, and of PTB ≤34 weeks at CL >25 mm.

Limitations: Small variations among operators may affect cervical length measurements. Patients with a short cervix were offered vaginal progesterone, which could affect the frequency of PTB.

Conclusion: In LR pregnant women, the prevalence of PTB ≤34 weeks was higher at CL 25-30 mm than at CL>30 mm. HR patients had a significantly higher prevalence of PTB at any CL than LR patients.