A simple model to predict the internal rotation
A total of 257(77.6%) women with a fetus in non-OA position during the latent phase underwent spontaneous rotation to OA position before delivery; however, 74 (22.4%) were persistent OP or OT position. According to the occurrence of internal fetal head rotation, the individual characteristics were similar besides maternal height (161.0±4.4 vs. 159.7±5.2 cm; P = 0.022) (Table S2). The multivariable logistic regression analysis demonstrated that the MLA (adjusted odds ratio [aOR]: 1.021, 95% confidence interval [CI]:1.008-1.034) and maternal height (aOR: 1.107, 95% CI:1.026-1.195) were the significant predictors of spontaneous fetal occiput rotation after adjusting for maternal age, gestational age, overweight, premature rupture of the membrane, labor induction, oxytocin augmentation and the use of epidural analgesia (Table S3). The final model that predicted internal fetal head rotation based on MLA at the first ultrasound scan and the maternal height was shown in Table 3, which obtained the AUC was 0.667 (95% CI, 0.583-0.751; P<0.001) (Figure 2). The calibration curve shows good agreement between predicted and observed probabilities for the occurrence of spontaneous fetal head rotation (Figure 3(a)).