The Value of Cervical Length Changes for the Prediction of Preterm Birth with Normal Mid-Trimester Cervical Length; a Prospective Longitudinal Study

Background The purpose of the study is to evaluate whether the change in cervical length (ΔCx) between the first and second trimesters can predict spontaneous preterm birth (sPTB) in women with a normal mid-trimester cervical length (CL > 25 mm). The study also aimed to explore the relationship between cervical dynamics and maternal characteristics -age, body mass index (BMI) and smoking status- on the prediction of sPTB in a low-risk cohort. Methods This prospective longitudinal cohort study included singleton pregnancies with first- and second-trimester transvaginal CL measurements > 25 mm. CL was measured at 11–14 (C1) and 18–24 weeks (C2). ΔCx was calculated as C2 − C1, and the rate of cervical change (slope) as (C2 − C1) divided by the gestational age interval between scans (mm/week). The primary outcome was sPTB < 37 weeks. Multivariable logistic regression was used to assess associations between CL parameters and sPTB, adjusting for maternal age, BMI and smoking. Nested models were compared to evaluate whether longitudinal cervical change added predictive value beyond second-trimester CL. Results A total of 2,317 women met inclusion criteria. The overall rate of sPTB was 8.24% before 37 weeks, and 0.52% delivered before 34 weeks. First-trimester CL was similar in term and preterm pregnancies (35.7 ± 4.6 mm vs 35.4 ± 4.7 mm, p = 0.44), as was second-trimester CL (37.4 ± 5.5 mm vs 36.7 ± 5.5 mm, p = 0.08). Mean ΔCx did not differ significantly between groups (1.68 ± 7.18 mm in term vs 1.29 ± 6.71 mm in preterm, p = 0.44), and the distribution of rate of cervical change (mm/week) overlapped substantially between term and preterm births. In multivariable analysis, neither ΔCx nor rate of cervical change was independently associated with sPTB, and adding slope did not improve model performance beyond second-trimester CL. Conclusions In women with a normal first- and mid-trimester cervix (CL > 25 mm), ΔCx-whether expressed as absolute difference or rate of change- does not meaningfully predict sPTB or enhance risk stratification beyond a single second-trimester CL measurement. These findings support continued reliance on mid-trimester transvaginal CL as the key sonographic screening tool in low-risk populations.