Maternal characteristics
A total of 634 nulliparous women were recruited, fifteen of whom were excluded because of incomplete data, and five withdrew due to discomfort during the ultrasound examination. Of the remaining 614 women, 524 (85.3%) women underwent vaginal delivery and 90 (14.7%) were required cesarean delivery. The percentage of women with fetuses in non-OA position at the first time of ultrasound examination was 53.9% (331 cases) in our study, as 159 (25.9%) cases were in the OP position and 172 (28.0%) were OT position. The main characteristics of the participants are shown in Table 1.