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

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

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.

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.

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.

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.