The base cohort included the participants who met the inclusion criteria during the study period, and the cohort will be followed up until their placentas are delivered. The case group will be the women who receive intrapartum caesarean delivery finally, while the control group will be those who vaginally deliver their live babies. Intrapartum caesarean delivery is defined as a final caesarean section for women who planned vaginally deliver and had entered the latent phase of labour process. The signs of entering the latent phase are regular and gradually increased uterus contractions that occur at 5–6 min intervals and last for at least 30 s, accompanied by the cervical canal efface, the dilatation of the cervix and the decreasing fetal presentation.24 Each cohort case will be matched with two controls on age (within 2 years), gestational week (within 2 weeks) and same parity. The technical protocol of this study is shown in figure 1.