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.