Predictive value of transvaginal ultrasound cervical length combined with cervical index dynamic monitoring for preterm birth risk in nulliparous women: a retrospective cohort study.

BACKGROUND

Preterm birth remains a leading cause of neonatal morbidity and mortality worldwide. Cervical length (CL) measured by transvaginal ultrasound is an established predictor, but single-timepoint measurements have limited accuracy, particularly in nulliparous women without prior obstetric history. This study evaluated whether dynamic monitoring of CL combined with cervical index (CI) improves preterm birth prediction in nulliparous women.

METHODS

This retrospective cohort study included 644 nulliparous women with singleton pregnancies who underwent serial transvaginal ultrasound cervical assessments at three timepoints: T1 (16-20 weeks), T2 (20-24 weeks), and T3 (24-28 weeks). CL, CI (calculated as [funnel length + 1]/CL), and rate of cervical change (ΔCL rate) were measured. Multivariable logistic regression identified independent predictors. Prediction models were compared using area under the receiver operating characteristic curve (AUC), and a nomogram was developed for clinical application.

RESULTS

Of 644 participants, 64 (9.9%) delivered preterm. Women who delivered preterm had significantly shorter CL at all timepoints (T2: 28.4 ± 7.2 vs. 36.6 ± 4.3 mm, p < 0.001) and faster cervical shortening rates (-1.12 ± 0.58 vs. -0.55 ± 0.28 mm/week, p < 0.001). The combined dynamic model incorporating CL, CI, and rate of change achieved superior predictive performance (AUC = 0.892, 95% CI: 0.849-0.935) compared to single CL measurement at T2 (AUC = 0.762, 95% CI: 0.698-0.826; p < 0.001). Independent predictors included CL at T2 (OR 1.86 per 5 mm decrease), ΔCL rate (OR 2.34 per 0.5 mm/week increase), maximum CI (OR 1.42), cervical funneling (OR 2.89), and premature rupture of membranes (OR 4.28). The nomogram demonstrated good calibration (Hosmer-Lemeshow χ2 = 8.42, p = 0.394) and clinical utility on decision curve analysis.

CONCLUSIONS

Dynamic monitoring of cervical parameters significantly enhances preterm birth prediction in nulliparous women compared to single-timepoint assessment. The combined model incorporating CL, CI, and rate of change provides excellent discrimination and may facilitate targeted preventive interventions. External validation in multicenter cohorts is warranted.