Model validation
The performance of the nomogram models in identification and calibration was evaluated. The discriminative ability and predictive ability of the model are evaluated through Harrell’s C-index, and external crowds are introduced to further evaluate the predictive value of the model. The calibration curve was analysed by drawing the predicted probability of the nomogram and the actual occurrence of SPTB. Restricted cubic splines were used to evaluate the correlation between the model’s predicted score and the risk value of preterm birth. And the Kaplan-Meier curve was used to compare the differences between different risk groups. ROC curve analysis was also used to compare the prediction performance of the nomogram prediction model and the univariate model.
Statistical analyses were all performed with R 3.6.0 software (R Foundation, Vienna, Austria). A two-sided P-value < 0.05 was considered to indicate statistical significance.