Validation of the nomogram
The discrimination power of the nomogram was determined using the ROC curve. Notably, the AUC value of the model of PTB < 37 weeks was 0.803 (95% CI 0.760–0.847), indicating good discrimination. This model showed a cut-off score of 0.389 with a sensitivity of 65.8% and a specificity of 84.0%. The AUC of the model of PTB < 34 weeks was 0.771 (95% CI 0.717–0.826), implying satisfactory discrimination. The cut-off score of this model was 0.227 with a sensitivity of 62.2% and a specificity of 83.4%.