Background: Results of American observational studies and one large, randomized trial
show that elective induction of labor among nulliparous women can reduce cesarean
delivery (CD) rates and suggest that gestational age at delivery may be a risk factor
for CD in pregnancies managed expectantly. However, data on the risk of CD at term
in ongoing pregnancies are sparse, especially in high-income countries, and further
information is needed to explore the external validity of these previous studies.
Objective: We studied the risk of CD for each gestational week of ongoing pregnancy
in nulliparous women with a singleton fetus in the cephalic presentation at term in
a French population.
Study design: This retrospective study was conducted in a perinatal network of 10
maternity units from January 1, 2016, through December 31, 2017, and included all
nulliparous women with a singleton fetus in the cephalic presentation who gave birth
at term (≥37 weeks + 0 day). From the start of term (37 completed weeks) and at the
start of each subsequent week of completed gestation (each week + 0 days), ongoing
pregnancy was defined as that of a woman who was still pregnant who gave birth at
any time after that date. For each week of gestation for these ongoing pregnancies,
CD rate was defined as the number of CDs performed in each ongoing pregnancy group
divided by the number of women in this group. Separate models for each week of gestation,
adjusted by maternal characteristics and hospital status, were used to compare the
CD risk between ongoing pregnancies and those delivered the preceding week. The same
methods were applied to subgroups defined according to the mode of labor onset. Odds
ratios were calculated after adjusting for maternal age and educational level, presence
of severe preeclampsia, and maternity unit status.
Results: The study included 11,308 nulliparous women, 22.5% (2544/11,308) of whom
had a CD. These rates remained stable for ongoing pregnancies at 37 weeks + 0 days,
38 weeks + 0 days, and 39 weeks + 0 days; the rates were 22.5% [95% confidence interval
(CI) 21.7%–23.2%]; 22.6% [21.8%–23.3%]; and 22.7% [21.9%–23.6%], respectively. The
risk of CD started to increase in ongoing pregnancies at 40 weeks + 0 days (24.3%
[95% CI 23.1%–25.4%]), and especially at 41 weeks + 0 days (30.7% [28.9%–32.5%]).
Similar trends were also shown for all modes of labor onset and in every maternity
unit. In univariate and multivariate analyses, ongoing pregnancy at or beyond 40 weeks + 0
days was associated with a higher risk of CD compared with pregnancy delivered the
previous week: 24.3% of ongoing pregnancies at 40 weeks + 0 days versus 19.9% of deliveries
between 39 weeks + 0 days and 39 weeks + 6 days. The odds ratios were 1.28 [95% CI
1.15–1.44] or 30.4% of ongoing pregnancies at 41 weeks + 0 days versus 1.73 [95% CI
1.51–1.96] or 19.6% of deliveries between 40 weeks + 0 days and 40 weeks + 6 days.
Conclusions: CD rates increased starting at 40 weeks + 0 days in ongoing pregnancies
regardless of the mode of labor onset.