Is gestational age at term a risk factor for ongoing pregnancies in nulliparous women: A prospective cohort study

Abstract

The results of American observational studies and 1 large, randomized trial show that elective induction of labor among nulliparous women can reduce cesarean delivery rates and suggest that gestational age at delivery may be a risk factor for cesarean delivery in pregnancies managed expectantly. However, data on the risk of cesarean delivery 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. This study aimed to evaluate the risk of cesarean delivery for each gestational week of ongoing pregnancy in nulliparous women with a singleton fetus in the cephalic presentation at term in a French population. This retrospective study was conducted in a perinatal network of 10 maternity units from January 1, 2016, to December 31, 2017, and included all nulliparous women with a singleton fetus in the cephalic presentation who gave birth at term (≥37 0/7 weeks of gestation). 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 and who gave birth at any time after that date. For each week of gestation for these ongoing pregnancies, the cesarean delivery rate was defined as the number of cesarean deliveries 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 cesarean delivery 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. The study included 11,308 nulliparous women, 2544 (22.5%) of whom had a cesarean delivery. These rates remained stable for ongoing pregnancies at 37 0/7, 38 0/7, and 39 0/7 weeks of gestation; the rates were 22.5% (95% confidence interval, 21.7-23.2), 22.6% (95% confidence interval, 21.8-23.3); and 22.7% (95% confidence interval, 21.9-23.6), respectively. The risk of cesarean delivery started to increase in ongoing pregnancies at 40 0/7 weeks of gestation (24.3%; 95% confidence interval, 23.1-25.4) and especially at 41 0/7 weeks of gestation (30.7%; 95% confidence interval, 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 0/7 weeks of gestation was associated with a higher risk of cesarean delivery than pregnancy delivered the previous week: 24.3% of ongoing pregnancies at 40 0/7 weeks of gestation vs 19.9% of deliveries between 39 0/7 weeks of gestation and 39 6/7 weeks of gestation. The odds ratios were 1.28 (95% confidence interval, 1.15-1.44) or 30.4% of ongoing pregnancies at 41 0/7 weeks of gestation vs 1.73 (95% confidence interval, 1.51-1.96) or 19.6% of deliveries between 40 0/7 weeks of gestation and 40 6/7 weeks of gestation. Cesarean delivery rates increased starting at 40 0/7 weeks of gestation in ongoing pregnancies regardless of the mode of labor onset.