This study is a randomized, controlled, observational study. 150 nulliparous women with estimated fetal weight ≥3500g at 39-40 weeks of gestation will be enrolled as subjects in the two groups is 1:1. In the experimental group, vaginal examination will be performed at 39 to 40 weeks to assess cervical conditions. If the bishop score <6, the balloon catheter combined with oxytocin induction will be planned at 40 weeks ±3 days. In the control group, one week to 41 weeks ±3 days will be expected. Vaginal examination will be performed again to evaluate cervical conditions. If the bishop score <6 points, and balloon catheter combined with oxytocin induction will be performed. After 96h, their final delivery mode will be recorded. In the following 42 days postpartum, their complications and the neonatal outcome will be followed up.

Inclusion Criteria: singleton head position pregnancy, negative Down's screening and /or NIPT screening, no malformation was found, with slight/without pregnancy complications -

Exclusion Criteria:twin or multiple pregnancy, scar uterus, with contraindications to vaginal delivery, estimated fetal weight >4500g

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