Objective: To study the risk of CD for each gestational week among ongoing pregnancies of nulliparous women at term. Design: A retrospective, population-based cohort study from January 1, 2016, through December 31, 2017 Setting: a French perinatal network of the Yvelines district, France Population: 11 308 nulliparous women with a singleton fetus in a cephalic presentation and delivered at term (≥37-week +0 day) Methods: for each week of gestation at term, we defined ongoing pregnancies as all

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... regnancies undelivered at the start of each week. Regression models adjusted by maternal characteristics and hospital status were used to compare the CD risk between ongoing pregnancies and the pregnancies delivered the preceding week. The same methods were applied to subgroups defined by the mode of labor onset. Main outcome measure: The caesarean delivery rate (CD) Results: Ongoing pregnancies > 40 weeks+0 days were associated with a higher risk of CD compared with pregnancies delivered the previous week: 24.3% in ongoing pregnancies ≥ 40 weeks +0 days versus 19.9% in deliveries between 39 weeks +0 days and 39 weeks+6 days (Odd ratio adjusted of 1.28, 95%CI [1.15-1.44]; 30.4% in ongoing pregnancies ≥ 41 weeks +0 days versus 19.6% in deliveries between 40 weeks +0 days and 40 weeks+6 days (OR 1.73, 95%CI [1.51-1.96]). This was also shown for all modes of labor onset and in every maternity unit. Conclusions: CD rates increased starting at 40 weeks +0 days in ongoing pregnancies, regardless of the mode of labor onset.