Longitudinal evaluation of cervical length for the prediction of mid-trimester cervical length in low-risk population

BACKGROUND: Preterm birth (PTB) remains a leading cause of perinatal mortality and long-term morbidity. While screening strategies primarily target high-risk populations, a substantial proportion of PTBs occur in low-risk women. Mid-trimester cervical length (CL) assessment may be implemented too late for effective prevention. Earlier evaluation in the first trimester may improve risk stratification. OBJECTIVE: To longitudinally evaluate cervical length from the first to the second trimester and assess its ability to predict mid-trimester CL and PTB in a low-risk population. METHODS: In this prospective cohort study, 292 low-risk pregnant women underwent transvaginal ultrasound at 11-14, 16-20, and 20-24 weeks of gestation. Cervical measurements included single-line CL (A), two-line components (B: internal os to cervical angulation; C: angulation to external os), and anterior (D) and posterior (E) cervical lip widths. Longitudinal changes were analyzed, and receiver operating characteristic (ROC) analysis was used to evaluate predictive performance for PTB <35 weeks. RESULTS: Median single-line CL (A) increased from 37.7 mm (29-58) at 11-14 weeks to 41.8 mm (28-66) at 20-24 weeks. This increase was primarily driven by elongation of the B component (13% increase, p<0.001), while the C component remained stable. First-trimester CL demonstrated limited predictive performance for PTB, with an area under the curve (AUC) of 0.61. CONCLUSION: Cervical length increases from the first to the second trimester in low-risk pregnancies, mainly due to changes in the proximal cervical segment. First-trimester CL measurement has limited value in predicting PTB, supporting the continued use of mid-trimester assessment for risk stratification.