Development and external validation of a nomogram for predicting spontaneous preterm birth in singleton pregnancies with a short cervix.

BACKGROUND

Spontaneous preterm birth (sPTB) is a major cause of neonatal morbidity and mortality. Women with a short cervix (≤25 mm) are at particularly high risk; prediction based solely on this single measurement is often insufficient. There is an urgent need for more accurate, externally validated tools to stratify risk in this high-risk population.

OBJECTIVE

To develop and externally validate a dynamic, multivariable nomogram for predicting individualized, time-dependent risks of sPTB in singleton pregnancies with a short cervix.

STUDY

DESIGN

We conducted a retrospective training cohort (n=591) of singleton pregnancies with a short cervix identified before 24 weeks of gestation from our Xicheng campus (2016-2022) and a prospective external validation cohort (n=215) from our Daxing campus (2024-2025). A multivariable Cox proportional hazards model was used to identify independent predictors. The model's performance was rigorously evaluated for discrimination (time-dependent area under the curve [AUC]), calibration (calibration plots and Brier scores), and clinical utility (decision curve analysis [DCA]).

RESULTS

Three independent predictors were incorporated into the final nomogram: a history of prior preterm birth (hazard ratios [HR]=3.94), cervical length (HR=0.91), and the gestational week at diagnosis (HR=1.18). The nomogram demonstrated good discrimination, with time-dependent AUCs for sPTB before 32, 34, and 37 weeks of 0.774, 0.740, and 0.724 in the training cohort, and 0.757, 0.734, and 0.719 in the external test cohort, respectively. The model was well-calibrated in both cohorts, confirmed by low Brier scores and nonsignificant Hosmer-Lemeshow tests. DCA showed a consistent net benefit across a wide range of risk thresholds. The model's performance was maintained despite the validation cohort representing a demographically distinct and higher-risk population.

CONCLUSION

This study established and externally validated a simple, clinically practical nomogram for predicting sPTB in singleton pregnancies with a short cervix. This robust, clinically applicable tool can aid in patient counseling and the stratification of high-risk pregnancies to guide personalized management strategies.