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

Thibault Quibel, Patrick Rozenberg, C. Duvillier, Camille Bouyer, Jean Bouyer

► To cite this version:

Thibault Quibel, Patrick Rozenberg, C. Duvillier, Camille Bouyer, Jean Bouyer. Is gestational age at term a risk factor for ongoing pregnancies in nulliparous women: A prospective cohort study. American Journal of Obstetrics & Gynecology MFM, 2023, 5 (2), 10.1016/j.ajogmf.2022.100808. hal-04122496

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1 **Condensation**

2 The risk of cesarean delivery increases in nulliparous women with a singleton fetus in the

3 cephalic presentation with ongoing pregnancy at 40 weeks and especially at 41 weeks.

4 **Short title:** Risk of cesarean delivery in ongoing pregnancy of nulliparous women at term.

5

6 AJOG at a Glance

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A. Why was this study conducted?

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10 A large, randomized trial reported a lower cesarean delivery rate associated with elective

11 induction of labor at 39 weeks of gestation in low-risk nulliparous women than in those

12 followed by expectant management. These findings raise the question whether active

13 management following induction of labor decreases cesarean delivery rates or whether

14 expectant management of a late birth increases the risk of cesarean delivery. It is unclear

15 whether the timing of delivery by itself constitutes a risk factor for cesarean delivery in

16 expectantly managed pregnancies or whether the active management of labor following

17 induction of labor decreases the cesarean delivery rate.

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B. Key findings

19 • The risk of cesarean delivery increased among nulliparous women with ongoing pregnancy at ≥40 weeks + 0 days.

21 • This increase was consistent regardless of the type of onset of labor but was most pronounced among nulliparous women in spontaneous labor.

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C. What does this add to what is known?

24 These findings support the hypothesis that gestational age beyond 39 weeks + 0 days in

25 pregnancies managed expectantly constitutes a risk factor for cesarean delivery and may

26 explain the benefits of elective induction of labor in low-risk nulliparous women.
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**Disclosure statement**

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The authors report no conflict of interest.

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**Keywords:** expectant management, induction of labor, cesarean delivery, ongoing pregnancies, nulliparous women, ongoing pregnancy, spontaneous labor

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**Abstract**

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**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.

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**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.

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**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.

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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.

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73 **Introduction**

74 Cesarean delivery (CD) rates have increased substantially over the past three decades in low-, 75 middle-, and high-income countries.¹,² This is a worldwide public health problem because CD 76 can lead to both short- and long-term health consequences for both the newborn and 77 mother.³,⁴ For decades, health-care professionals, researchers, and policy-makers have failed 78 to provide a clear answer about the “optimal” population-level frequency of CD. Current 79 research focuses on optimizing CD use by selecting targeted populations to try to reduce or at 80 least stabilize the number of unnecessary CDs.⁵⁻⁸

81 Many drivers play a role in the excess use of CD. Some are related to the clinical status of the 82 mother or the fetus; beliefs of the mother and of society more broadly; beliefs, preferences, 83 and experience of health-care professionals; and the organization of health-care systems, 84 facilities, and funding mechanisms.⁹⁻¹¹ One relevant question raised recently is that of the 85 optimal timing of delivery of term pregnancies. A large, randomized trial of low-risk 86 nulliparous women found that induction of labor at 39 weeks was associated with a decreased 87 risk of CD, which suggests that expectant management past 39 weeks may be a risk factor for 88 CD.¹² Observational comparative studies from the US have reported similar findings, 89 although the effect on CD rates is not as clear in Western and Nordic European countries.¹³⁻²⁰ 90 Most studies have focused on the elective induction of labor as a potential protection act 91 against the need for CD and **have not documented** the risk of CD in pregnancies managed 92 expectantly at term. Using data from the ARRIVE trial, Tita et al. analyzed the weekly CD 93 rates in pregnancies managed expectantly.²¹ They found that the CD rates increased 94 significantly week after week when expectant management was decided at 39 weeks. 95 However, they had analyzed the CD rates retrospectively during each week of gestation, and 96 this may not have been the most effective way to study the risk of CD according to gestational 97 age in ongoing pregnancies (OPs); that is, as if the pregnancies were managed expectantly.
The timing of delivery itself may constitute a risk factor for CD, especially in full- and late-
term OPs. That is, continuation of the pregnancy may increase the risk of requiring a CD. The
aim of this study was to evaluate the risk of CD for each additional week of gestation in
nulliparous women with an OP that has reached 37 weeks + 0 days.

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104 Methods

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106 Design of the study and study population

107 This study used data from a population-based cohort of births from January 1, 2016, through
108 December 31, 2017, in the Maternités en Yvelines et Périnatalité Active (MYPA) perinatal
109 network. This network includes all maternity units in the district of Yvelines (west of Paris)
110 and manages more than 16,000 deliveries per year. It comprises 10 maternity facilities
111 (referred to here as A through J), including one academic maternity unit (A) that is affiliated
112 with the Université Versailles-Saint Quentin. Half the establishments are public (A, B, C, E,
113 and H) and half private (D, F, G, I, and J). Only two (A and B), which both include a neonatal
114 intensive care unit (NICU), manage all types of pregnancies. The other eight (C-J) are level-1
departments intended to serve only low-risk pregnancies.
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117 Data source and study exposure variables

118 The CoNaissance Program 78 was initially established in 2008 in collaboration with local
119 authorities (Conseil Départemental des Yvelines), l'Agence Régionale de Santé Île-de-France
120 (Regional Health Agency), and Inserm Unit 953 to monitor maternal and perinatal morbidity
121 and mortality prospectively in a French perinatal network. Data for all deliveries occurring
122 after 24 weeks of gestation were recorded continuously in two health certificates collected
123 locally. The first was a health certificate for infants born in the network's maternity units
124 during a nationally mandatory medical examination performed within eight days after birth,
125 usually completed in the maternity ward. The second was a complementary health certificate
126 developed specifically within the MYPA network that reported additional data such as severe
127 maternal morbidity, familial social characteristics, fetal deaths, and terminations of pregnancy
128 for medical reasons at and after 22 weeks of gestation.
We restricted this study to nulliparous women with a singleton fetus in the cephalic presentation and a pregnancy still ongoing at 37 weeks + 0 days. In accordance with French guidelines, pregnancy dating was performed using the crown-rump length measured during the first trimester ultrasound scan, which is performed routinely in France.

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## 134 Ethics approval

The National Committee for Data Protection (Commission Nationale de l'Informatique et des Libertés, registration number 1295794) approved the study, which was conducted in accordance with French legislation. Under French law, the study was exempt from informed consent requirements because patients received standard care and because the data set contained no information that could enable patient identification. Similarly, ethics committee approval was not required because the study used an anonymized database and had no influence on patient care.

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## 143 Cesarean delivery rate in ongoing pregnancies

Our objective was to examine the risk of CD when expectant management has been decided. However, in observational studies, it is difficult to identify the clinician's intention to manage a pregnancy expectantly given that decisions for elective delivery involve induction of delivery for both medical and nonmedical reasons. Moreover, plans for expectant management of a pregnancy may change as it continues. Therefore, we used OP as a proxy for expectant management. At the start of each gestation week (those at + 0 days), an OP was identified as a woman who remained pregnant and who gave birth at any time thereafter. For example, the OP group at the start of the study period at 37 weeks included all women who had not given birth before 37 weeks + 0 days but who gave birth on or after that day. The OP
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group became progressively smaller each week and was not limited to the pregnancies
delivered in any given week.

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## French policy for elective induction of labor

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Indications for the induction of labor have not been established in France. However, elective induction remains subject to conditions defined by the Haute Autorité de Santé (HAS), which requires a gestational age ≥39 weeks + 0 days and a favorable cervix. Elective induction of labor accounts for 2% of all inductions in nulliparous women in France.²²

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## Statistical analysis

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Descriptive statistics were used to provide an overview of the study population and estimate the global risk of CD in nulliparous women for the women with an OP each week. Maternal characteristics and maternity unit status (public or private) were compared for each gestational age at delivery. The chi-square test or Fisher's exact test was used to analyze categorical variables and Student's t test was used for continuous variables.

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For a given week of pregnancy, we considered two groups of pregnancies (see Figure 1): those delivered in the previous week and those not delivered that week (i.e., OPs), and we compared the percentage of CDs between each group. For the first group, delivery (CD or not) occurred in the preceding week and, for the second group it occurred at any time from the first day of the week to the end of the study period (which could be several weeks later). In both groups, all deliveries were considered, regardless of the mode of onset of labor (spontaneous, induced, or prelabor CD). The difference in CD rates between delivered and OPs were computed with 95% confidence intervals (CIs) and compared using the chi-square test, and are reported as odds ratios (ORs). In a previous study, we showed that CD was associated with maternal age, educational level, severe preeclampsia, and maternity unit

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status. These factors were selected a priori in the logistic regression for adjustment of the OR
of CD.²³

We also sought to ascertain how the CD rate evolves with gestational age according to the
mode of labor onset. CD rates among nulliparous women who gave birth after spontaneous
labor were calculated for each gestational week of OPs by dividing the number of CDs after
spontaneous labor by all births that occurred after spontaneous labor among the women with
OPs. We used the same procedure for CDs after the induction of labor. Similarly, for both
modes of onset of labor, we computed the ORs and adjusted ORs (aORs) with 95% CIs for
CD and compared these between OPs and pregnancies delivered the week before. All
analyses were performed using R Studio (version 1.0.136). Significance was defined as P ≤
0.05.

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190 **Results**

191 During the 2-year study period, 32,766 women gave birth at a gestational age ≥24 weeks in
192 one of MYPA's 10 maternity units. The overall CD rate in the network was 25.8%. Our study
193 included 11,308 nulliparous women with a singleton fetus in the cephalic presentation who
194 did not give birth before 37 weeks + 0 days. The CD rate in this population was 22.5% and
195 varied between maternity units (range 15.4%-40.2%, P < 0.05). The rates of induction of
196 labor and prelabor CD were 28.1% (range 20.6%-41.4%, P < 0.05) and 4.4% (1.3%-19.2%, P
197 < 0.05), respectively, and differed significantly between units. As for the overall CD rates,
198 CD rates after spontaneous labor varied between maternity units (range: 8.4%-21.3%, P <
199 0.001), but CD rates after induction of labor did not differ significantly (range 29.1%-39.7%,
200 not significant) (Table 1).

201 The characteristics of the study population are summarized in Table 2. Women who gave
202 birth at ≥40 weeks + 0 days were less likely to be older than 35 years, to have completed high
203 school, or to have had a pregnancy uncomplicated by severe hypertensive disorder or diabetes
204 mellitus. Delivery in a private maternity unit was most frequent at 38 or 39 weeks of
205 gestation.

206 A flow chart of the 11,308 OPs at 37 weeks + 0 days is presented in Figure 1. Nearly half the
207 nulliparous women had not given birth at 40 weeks + 0 days, and 20% had a late-term
208 pregnancy (≥41 weeks + 0 days). An explanation of how the CD risk by gestational age can
209 be interpreted in different ways, depending on how the CD rate is calculated, is presented in
210 Figure 1. The left part of Figure 1 shows the ratio between the number of CDs and all
211 deliveries in a given week (i.e., with delivered pregnancies as the denominator). Taking the
212 week of gestation into account by using this denominator showed that CD rates were lowest
213 for deliveries in weeks 39 (between 39 weeks + 0 days and 39 weeks + 6 days) and 40
214 (between 40 weeks + 0 day and 40 weeks + 6 days), and highest in week 41. These results are
explained by the high rate of spontaneous labor in weeks 39 and 40, whereas the CD rate in week 41 (and later) was associated with a high proportion of induced labor. Therefore, when the denominator is the number of pregnancies delivered a given week of gestation, the CD rate seemed to vary according to the proportions of spontaneous and induced labor.

The CD rates in the OPs (with OPs as the denominator) differed when pregnancies reached full term, and the CD rate increased as pregnancies reached 40 weeks. More precisely, the CD rates in OPs were influenced by the proportion of pregnancies still to be delivered at late term (≥41 weeks + 0 day) and the CD rates observed in this late population. These results were found in each maternity unit, regardless of its overall CD rate or overall induction rate (Appendix A).

The OR used to compare the CD rates in OPs with the deliveries in the preceding week was significantly higher after 39 weeks. This result remained consistent after adjustment for maternal age, educational level, severe preeclampsia, and maternity unit status (Table 3), and seemed to be consistent across all maternity units (Figure 2).

In the group of women who gave birth after spontaneous labor, CD rates were stable at around 13.5% for OPs that reached 37 weeks + 0 days, 38 weeks + 0 days, and 39 weeks + 0 days.

However, the rates began to increase starting at 40 weeks and reached 20% in late-term pregnancies (Table 4). Among the women with an OP who gave birth after the induction of labor, the CD rate also increased with each week of gestation (Table 5). These findings were consistent across all maternity units (Figure 2).

## Discussion

## Main findings
This analysis of more than 11,000 deliveries in 10 maternity units of a perinatal network found a higher risk of CD among nulliparous women with OP beyond 39 weeks + 6 days.

This finding was observed in every maternity unit in the network. These findings are explained by the number of deliveries at ≥41 weeks + 0 day among the OPs regardless of the mode of labor onset.

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## Strengths and limitations

The strength of this study is the large sample from a well-defined population involving an entire perinatal network. The quality of the data is high because they were obtained prospectively by trained personnel and the database was checked by regular audits.²⁴

Although the overall CD rate in this perinatal network (26.5%) differs from that of other perinatal networks and other countries, the external validity is supported by the results of this study. Importantly, the higher risk in the perinatal network for OPs at or beyond 40 weeks + 0 days was similar in every maternity facility in this network even though their overall rates of CD and labor induction varied substantially. The CD rate for ongoing pregnancies at 39 weeks + 0 days was 22.7%, which is similar to that in the expectant management group in the ARRIVE trial (22.2%) and in another observational study.¹² In a comparison of CD rates for nine maternity units from different countries, Brennan et al. found a mean 21% CD rate (range: 13.8%–30.6%) for nulliparous women with a fetus in the cephalic presentation who gave birth at term (≥37 weeks + 0 day); the 22.5% rate observed in our study is only slightly higher.²⁵

Our study has several limitations. First, the statistical analysis used to compare the CD rates in OPs deserves an explanation. Cumulative probabilities and life table analysis are usually appropriate for exploring the risk of an adverse outcome that varies across time, such as the CD risk in our study. Although these models have been used for evaluating the risk of
264 stillbirth according to gestational age, studies that have used these models have not provided
265 comprehensive data on the risk of CD for OPs.²⁶,⁷⁴ Standard regression models would have
266 been inappropriate for comparing CD rates for each week of OPs in our study because
267 patients could have been classified into nested groups; for example, a woman delivered at 39
268 weeks + 3 days may also have been classified into the OP group (≥37 weeks + 0 days) as well
269 as the 38 weeks + 0 days and 39 weeks + 0 days groups. Therefore, the only way to evaluate
270 the CD risk for OPs was to compare CD rates in OPs with those for patients who gave birth
271 the week before. As noted above, the results were consistent across all 10 maternity units in
272 our network; that is, the CD risk increased with gestational age and specifically for late-term
273 deliveries (at 40 weeks + 0 days). These findings support our hypothesis that gestational age
274 at delivery may be a risk factor for CD, especially for OPs at late term.

275 The objective of the current study was to estimate the risk of CD for women for whom
276 expectant management was planned. This study is subject to the limitations of all
277 observational studies that use cross-sectional data to address a longitudinal question. Because
278 no data were available about the indications for the induction of labor, it was impossible to
279 know whether the clinician had planned to manage the pregnancy expectantly or had intended
280 to induce labor electively. In France, the national authority for health (HAS) recommends that
281 elective delivery for nonmedical reasons (i.e., maternal request) not be performed until at least
282 39 weeks of gestation and only with a favorable cervix.²⁸ Epidemiological data for elective
283 early delivery for nonmedical reasons in France are sparse, but the HAS conditions for
284 induction that is not medically indicated are rarely present in nulliparous women, even after
285 39 weeks.²⁶⁹ We therefore considered OP as a proxy for expectant management, notably for
286 OPs at and after 39 weeks + 0 days.

287 Finally, the occurrence of certain confounders of the risk of CD, such as preeclampsia and
288 macrosomia, may increase with gestational age. Unfortunately, the indications for induction
of labor and the occurrence of these confounders were not available in this population. We
found that the CD rates increased for each week of gestation in undelivered pregnancies and
that the occurrence of such complications may be part of the explanation for this increase.
This is not inconsistent with our results showing that, regardless of the cause, allowing an OP
to continue to full or late term may increase the risk of having a CD.

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## Interpretation in light of other evidence

The question of the optimal timing of delivery was initially raised to improve perinatal
morbidity and mortality because late-term pregnancies are associated with higher risks of
stillbirth and neonatal morbidity.³⁰⁻³² The ARRIVE trial examined whether, among low-risk
nulliparous women, elective induction of labor at 39 weeks of gestation compared with
expectant management could reduce the risk of the composite outcome of perinatal morbidity.
The difference between the two groups in the primary criterion did not reach significance, but
the authors found a significant reduction in the CD rate in the elective induction of the labor
group compared with the group managed expectantly (18.6% vs 22.2%; RR 0.84; 95% CI
0.76–0.93; P < 0.001).¹² Although the ARRIVE trial was based on a large population (6108
nulliparous women) and took place in many academic and community hospitals, its external
validity and its generalizability to other institutions and other countries are still debated,
especially given that the CD rates in the United States are considered among the highest
among high-income countries. The proportion of CDs in the expectant management group in
the ARRIVE trial appeared to be high because many perceive expectant management to be
associated with spontaneous labor, which is the mode of labor onset associated with the
lowest CD rates. In our study, only two-thirds of OPs at 39 weeks + 0 days were delivered
after a spontaneous labor, and this rate declined to <50% for prolonged pregnancies (≥41
weeks + 0 days).
314 Our study also provides explanations about the benefit of a strategy of elective induction of
315 labor compared with expectant management. The results of the ARRIVE trial raise the
316 question whether active management of labor that follows induction of labor decreases the
317 CD rates or whether expectant management until a late birth increases the CD rate. Induction
318 of labor is often associated with the active management of labor, which includes routine
319 amniotomy and use of the intravenous drug oxytocin to increase contractions of the uterus. In
320 the last Cochrane review, after exclusion of a study that included many postrandomization
321 exclusions, active management of labor was associated with a significant decrease in CS rate
322 compared with routine care (RR 0.77; 95% CI 0.63–0.94).³³

323 By contrast, expectant management can increase the risk of adverse outcomes, such as
324 macrosomia, chorioamnionitis, meconium staining, and hypertensive disorders. The increased
325 risk of these outcomes in OPs beyond 38 weeks + 0 days may partly explain the increase in
326 CD rate in full- and late-term pregnancies.¹⁹,³⁴ Although these possible confounders were not
327 included in our analysis, we found that regardless of the cause, allowing an OP to progress to
328 full or late term increased the risk of having a CD. Because the CD rate increases with
329 gestational age and when spontaneous labor occurs, the latter hypothesis seems to be the most
330 likely.

331 Surprisingly, how CD rates progress in terms of the onset of labor has not been explored
332 thoroughly. Some clinicians may believe that the gestational age has no influence on the risk
333 of CD during spontaneous or induced labor. However, in our study, the CD risk increased for
334 both modes of labor onset and was most pronounced for spontaneous labor.

335 This study was not designed and does not attempt to define the optimal gestational age for
336 delivery but was performed to examine the risk of CD in OPs as the term advances. In the
337 past few years, during which the potential benefit of a policy of elective induction of labor on
338 CD rates has been explored, issues affecting the population of expectantly managed
339 pregnancies have not been addressed adequately. OP can serve as a proxy for expectant
340 management and is an appropriate parameter for investigating adverse outcomes in OPs.
341 Through its simple method of deconstructing CD rates for each week of OPs, the current
342 study provides further information about how the risk of CD increases in OPs, in particular, in
343 full-, and late-tem pregnancies (≥40 weeks + 0 day).

344

345 **Conclusion**

346 The risk of CD increases as the OP reaches full or late term in nulliparous women. Further
347 research is needed to investigate if this risk also occurs in OPs of parous women and to
348 ascertain whether the influence of gestational age at delivery is similar or different in these
349 women.

350

351 *Disclosure of interest:* The authors of this manuscript (TQ, CB, CD, JB, PR) have no conflicts
352 of interest to disclose.

353 *Author contributions:* TQ developed the research question, facilitated analysis of the samples,
354 designed the study and analytic plan, analyzed and interpreted the data, and drafted the
355 manuscript. CB participated in the data collection. PR, CD, and JB contributed to writing the
356 manuscript. All authors edited, read, and approved the final manuscript.

357 *Funding:* There is no funding to report for this study.

358

359
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<table><caption>Table 1: Percentages of modes of onset of labor and cesarean delivery (CD) rates for each perinatal network maternity (Mat) unit</caption><thead><tr><th></th><th>Perinatal network</th><th>Mat A</th><th>Mat B</th><th>Mat C</th><th>Mat D</th><th>Mat E</th><th>Mat F</th><th>Mat G</th><th>Mat H</th><th>Mat I</th><th>Mat J</th><th>P</th></tr></thead><tbody><tr><td>Number of deliveries</td><td>11,308</td><td>2965</td><td>1263</td><td>1693</td><td>1125</td><td>599</td><td>1087</td><td>932</td><td>468</td><td>662</td><td>514</td><td></td></tr><tr><td>Percentages of spontaneous labor</td><td>67.5%</td><td>67.0%</td><td>68.8%</td><td>68.2%</td><td>71.6%</td><td>66.6%</td><td>60.2%</td><td>73.2%</td><td>68.8%</td><td>71.9%</td><td>54.9%</td><td>&lt;0.001</td></tr><tr><td>Percentages of induction of labor</td><td>28.1%</td><td>29.7%</td><td>29.2%</td><td>30.4%</td><td>26.8%</td><td>32.1%</td><td>20.6%</td><td>21.7%</td><td>27.6%</td><td>22.2%</td><td>41.4%</td><td>&lt;0.001</td></tr><tr><td>Percentages of prelabor CD</td><td>4.4%</td><td>3.2%</td><td>2.0%</td><td>1.4%</td><td>1.5%</td><td>1.3%</td><td>19.2%</td><td>5.2%</td><td>3.6%</td><td>5.9%</td><td>3.7%</td><td>&lt;0.001</td></tr><tr><td>Overall CD Rate</td><td>22.5%</td><td>21.5%</td><td>20.3%</td><td>18.3%</td><td>15.4%</td><td>20.4%</td><td>40.2%</td><td>24.2%</td><td>24.4%</td><td>24.2%</td><td>20.6%</td><td>&lt;0.001</td></tr><tr><td>CD rate among women with spontaneous labor</td><td>13.4%</td><td>13.0%</td><td>12.0%</td><td>11.8%</td><td>8.4%</td><td>13.8%</td><td>21.3%</td><td>15.7%</td><td>16.5%</td><td>15.3%</td><td>9.6%</td><td>&lt;0.001</td></tr><tr><td>CD rate among women with induction of labor</td><td>32.4%</td><td>32.2%</td><td>34.7%</td><td>30.1%</td><td>29.1%</td><td>30.1%</td><td>39.7%</td><td>36.7%</td><td>34.2%</td><td>33.3%</td><td>28.2%</td><td>0.165</td></tr></tbody></table>

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<table><caption>Table 2: Risk of cesarean delivery (CD) per week of gestation</caption><thead><tr><th>Gestational age</th><th>Overall number of deliveries</th><th>Overall number of CDs</th><th>CD rate (%), 95% CI</th></tr></thead><tbody><tr><td>Ongoing pregnancies (OPs) 37 weeks + 0 days</td><td>11,308</td><td>2544</td><td>22.5% (21.7%-23.2%)</td></tr><tr><td>OPs 38 weeks + 0 days</td><td>10,617</td><td>2397</td><td>22.6% (21.8%-23.3%)</td></tr><tr><td>OPs 39 weeks + 0 days</td><td>8944</td><td>2034</td><td>22.7% (21.9%-23.6%)</td></tr><tr><td>OPs 40 weeks + 0 days</td><td>5746</td><td>1396</td><td>24.3% (23.1%-25.4%)</td></tr><tr><td>OPs 41 weeks + 0 days</td><td>2420</td><td>744</td><td>30.7% (28.9%-32.5%)</td></tr></tbody></table>

CI: confidence interval; OPs: ongoing pregnancies

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<table><caption>Table 3: Crude odds ratios (ORs) and adjusted odds ratios (aORs) and their 95% confidence intervals (CIs) for CD rates for ongoing pregnancies (OPs) compared with those for women who gave birth in the preceding week of gestation</caption><thead><tr><th></th><th>CD rates for OP</th><th>CD rates in women who gave birth in the preceding week of gestation</th><th>OR (95% CI)</th><th>P</th><th>aOR (95% CI)</th><th>P</th></tr></thead><tbody><tr><td>Deliveries for OPs ≥37 weeks + 0 days</td><td>22.5% (2544/11,308)</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Deliveries for OPs ≥38 weeks + 0 days vs deliveries between 37 weeks + 0 days and 37 weeks + 6 days</td><td>22.6% (2397/10,617)</td><td>21.3% (147/691)</td><td>1.08 (0.89–1.30)</td><td>0.42</td><td>1.10 (0.88–1.41)</td><td>0.42</td></tr><tr><td>Deliveries for OPs ≥39 weeks + 0 days vs deliveries between 38 weeks + 0 days and 38 weeks + 6 days</td><td>22.7% (2034/8944)</td><td>21.8% (364/1673)</td><td>1.07 (0.96–1.19)</td><td>0.22</td><td>1.09 (0.94–1.24)</td><td>0.21</td></tr><tr><td>Deliveries for OPs ≥40 weeks + 0 days vs deliveries between 39 weeks + 0 days and 39 weeks + 6 days</td><td>24.3% (1396/5746)</td><td>19.9% (638/3198)</td><td>1.23 (1.12–1.34)</td><td>&lt;0.001</td><td>1.28 (1.15–1.44)</td><td>&lt;0.001</td></tr><tr><td>Deliveries for OPs ≥41 weeks + 0 days vs deliveries between 40 weeks + 0 days and 40 weeks + 6 days</td><td>30.4% (744/2420)</td><td>19.6% (652/3326)</td><td>1.74 (1.58–1.93)</td><td>&lt;0.001</td><td>1.73 (1.51–1.96)</td><td>&lt;0.001</td></tr></tbody></table>

The comparisons are pairwise; that is, CD rates for OP ≥38 weeks + 0 days compared with CD rates in pregnancies delivered between 37 weeks + 0 days and 37 weeks + 6 days.

To estimate the risk of delivery at a given week of gestational age, the percentage was compared for CDs for OPs with that of CDs in the preceding week. In the mixed model, the associations between CD rates and gestational age were adjusted for maternal age, educational level, severe preeclampsia, and maternity unit status.
<table><caption>Table 4: Crude odds ratios (ORs) and adjusted ORs (aORs) and their 95% confidence intervals (CIs) for D rates for ongoing pregnancies (OPs) compared with those for women who gave birth in the preceding week of gestation among women delivered after spontaneous labor</caption><thead><tr><th></th><th>CD rates for OPs</th><th>CD rates for women who gave birth in the preceding week of gestation</th><th>OR (95% CI)</th><th>aOR (95% CI)</th></tr></thead><tbody><tr><td>Deliveries for OPs ≥37 weeks + 0 days</td><td>13.3% (1020/7642)</td><td></td><td></td><td></td></tr><tr><td>Deliveries for OPs ≥38 weeks + 0 days vs deliveries between 37 weeks + 0 days and 37 weeks + 6 days</td><td>13.5% (978/7224)</td><td>10% (42/418)</td><td>1.41 (1.02–1.96)</td><td>1.44 (1.00–2.17)</td></tr><tr><td>Deliveries for OPs ≥39 weeks + 0 days vs deliveries between 38 weeks + 0 days and 38 weeks + 6 days</td><td>14.0% (862/6148)</td><td>10.8% (116/1076)</td><td>1.34 (1.10–1.66)</td><td>1.40 (1.11–1.80)</td></tr><tr><td>Deliveries for OPs ≥40 weeks + 0 days vs deliveries between 39 weeks + 0 days and 39 weeks + 6 days</td><td>16.0% (617/3850)</td><td>10.7% (245/2298)</td><td>1.59 (1.37–1.87)</td><td>1.60 (1.33–1.92)</td></tr><tr><td>Deliveries for OPs ≥41 weeks + 0 days vs deliveries between 40 weeks + 0 days and 40 weeks + 6 days</td><td>20.0% (234/1171)</td><td>14.3% (383/2679)</td><td>1.49 (1.25–1.79)</td><td>1.62 (1.34–1.97)</td></tr></tbody></table>

The comparisons are pairwise; that is, CD rates for OP ≥38 weeks + 0 days compared with CD rates in pregnancies delivered between 37 weeks + 0 days and 37 weeks + 6 days.

To estimate the risk of delivery at a given week of gestational age, the percentage was compared for CDs for OPS with that of CDs in the preceding week. In the mixed model, the associations between CD rates and gestational age were adjusted for maternal age, educational level, severe preeclampsia, and maternity unit status.
<table><caption>Table 5: Crude odds ratios (ORs) and adjusted ORs (aORs) and their 95% confidence intervals (CIs) for CD rates for ongoing pregnancies (OPs) compared with those for women who gave birth in the preceding week of gestation among women delivered after induction of labor</caption><thead><tr><th></th><th>CD rates for OPs</th><th>CD rates for women who gave birth in the preceding week of gestation</th><th>OR (95% CI)</th><th>aOR (95% CI)</th></tr></thead><tbody><tr><td>Deliveries for OPs ≥37 weeks + 0 days</td><td>32.3% (1023/3165)</td><td></td><td></td><td></td></tr><tr><td>Deliveries for OPs ≥38 weeks + 0 days vs deliveries between 37 weeks + 0 days and 37 weeks + 6 days</td><td>32.8% (965/2939)</td><td>38.5% (105/273)</td><td>1.42 (1.05-1.95)</td><td>1.43 (1.01-2.04)</td></tr><tr><td>Deliveries for OPs ≥39 weeks + 0 days vs deliveries between 38 weeks + 0 days and 38 weeks + 6 days</td><td>34.2% (843/2467)</td><td>41.4% (395/900)</td><td>1.44 (1.20-1.87)</td><td>1.43 (1.11-1.85)</td></tr><tr><td>Deliveries for OPs ≥40 weeks + 0 days vs deliveries between 39 weeks + 0 days and 39 weeks + 6 days</td><td>36.4% (640/1759)</td><td>41.3% (267/647)</td><td>1.43 (1.19-1.74)</td><td>1.41 (1.13-1.77)</td></tr><tr><td>Deliveries for OPs ≥41 weeks + 0 days vs deliveries between 40 weeks + 0 days and 40 weeks + 6 days</td><td>37.4% (441/1180)</td><td>40.8% (510/1249)</td><td>1.13 (0.91-1.39)</td><td>1.11 (0.87-1.42)</td></tr></tbody></table>

The comparisons are pairwise; that is, CD rates for OP ≥38 weeks + 0 days compared with CD rates in pregnancies delivered between 37 weeks + 0 days and 37 weeks + 6 days.

To estimate the risk of delivery at a given week of gestational age, the percentage was compared for CDs for OPs with that of CDs in the preceding week. In the mixed model, the associations between CD rates and gestational age were adjusted for maternal age, educational level, severe preeclampsia, and maternity unit status.
Figure 1: Deliveries from January 1, 2016, to December 31, 2017, in nulliparous women with a singleton fetus in the cephalic presentation with an ongoing pregnancy at 37 weeks + 0 days. The panel on the left shows the CDs for each week of delivery.

Figure 2: Forrest plot of adjusted odds ratios (aORs) and their 95% confidence intervals for CD rates for ongoing pregnancies compared with those for women who gave birth in the preceding week of gestation within each maternity unit of the perinatal network (A and B: aORs for the overall CD rates; C and D: aORs for the CD rates after spontaneous labor; E and F: aORs for the CD rates after induction of labor).