METHODS
This prospective, longitudinal observational, single-center study was conducted in the Women’s Hospital at Zhejiang University School of Medicine in Hangzhou, China, between May 2020 and November 2020. Nulliparous women with a singleton pregnancy in a cephalic presentation at term (37 or more full weeks of gestation) attempting vaginal birth were eligible for the study. Women were excluded if they were under 18 years of age, had intrauterine death or had emergencies (e.g., premature placental abruption, uterine rupture, cardiac disease of New York Heart Association (NYHA) grade 3 or 4, eclampsia). We also excluded the cases if they required cesarean deliveries purely because of fetal distress. The study was approved by the local ethics and research committees (IRB-20200148-R). Written consent was obtained from the women who agreed to participate.