Predictive factors associated with SPTB at < 32 weeks
In the training group, we conducted univariate and multivariate regression analysis to detect the correlations between clinical variables and probabilities of preterm delivery before 28 weeks, 32 weeks, and 34 weeks by applying the AIC-based backward procedure repectively (Table 1). Then we constructed three ROC curves for predicting SPTB according to the results of multivariate analysis. By comparing the AUCs, we found that the predicted value for SPTB at < 32 weeks was the highest (Supplementary Figure 1). After comprehensively considering the predictive power and the number of positive cases of PTB before the there gestational weeks(the number of positive cases who delivered before 28 weeks was significantly less than 10 times that of multivariate meaningful variables), we finally chose to establish a predictive model for predicting PTB at < 32 weeks. Multivariate logistic regression analysis (<32 weeks) showed that nulliparity, monochorionicity, prepregnancy BMI, previous preterm birth or late abortion, cervical funneling and shorter cervical length were independent risk factors for SPTB at < 32 weeks.