In this large, multicenter study in the United States, more than one third of elective repeat cesarean deliveries at term were performed before 39 weeks of gestation. As compared with deliveries at 39 weeks, these early deliveries were associated with a significantly increased risk of a composite outcome that included neonatal death or any adverse outcome, as well as increased risks of individual neonatal adverse outcomes that included respiratory complications and admission to the neonatal ICU. The risks of individual adverse neonatal outcomes were higher for delivery at 37 weeks (by a factor of 1.8 to 4.2) than for delivery at 38 weeks (by a factor of 1.3 to 2.1) relative to delivery at 39 weeks of gestation. Delaying delivery beyond 40 weeks was also associated with increased rates of neonatal adverse outcomes. However, only one neonatal death and no cases of hypoxic–ischemic encephalopathy or necrotizing enterocolitis were observed in this select cohort of low-risk term births.
The validity of these findings depends on our ability to have correctly identified and excluded deliveries for which there were indications for delivery before 39 weeks. Specifically trained and certified personnel scrutinized the charts of all women in the registry and their infants for indications for cesarean delivery. Our criteria for elective cesarean delivery were stringent. Although fetal growth restriction was not captured as a specific indication for cesarean delivery, our results remained unchanged after the exclusion of potentially growth-restricted neonates (birth weight <2500 g). It is possible that some earlier term deliveries may have been performed because of maternal perception of reduced fetal movement (a sign of potential fetal compromise). Although such patients should generally undergo antepartum fetal testing for signs suggesting fetal compromise20
(which would be coded as an indication for delivery), clinicians may forgo fetal testing before delivery at term. This might have affected our results in the unlikely event that more of such deliveries occurred before than after 39 weeks.
Also important is the accuracy with which we assigned gestational age. Our results were materially unchanged in analyses restricted to infants with gestational age estimated by first- or second-trimester ultrasound examination, rather than by third-trimester ultrasound examination or last menstrual period alone. The increase in mean birth weight as the assigned gestational age advanced provides further support for the accuracy of our dating.
Some limitations of the study should be noted. Deliveries that occurred before 39 weeks of gestation but after positive results of tests of lung maturity would not be considered inappropriately early3,4
; however, we did not have information about testing for lung maturity. One retrospective study of cesarean delivery at a single U.S. center, although potentially nonrepresentative of the general population, showed that 22% of elective cesarean deliveries were performed early without any tests for fetal lung maturity.21
Further study is warranted to assess whether an increased rate of adverse outcomes in deliveries occurring before 39 weeks is explained entirely by failure to test for fetal lung maturity before delivery, or whether testing indicating “maturity” does not fully protect against an increased rate of adverse outcomes in these early births.
Our study design did not allow us to assess whether there was an increase in stillbirths that was associated with delaying delivery until at least 39 weeks. The risk of unexplained stillbirth has been reported to be no greater than 0.2 of 1000 births at 37 weeks and 0.5 of 1000 births at 38 weeks among Scottish and Canadian cohorts of women who had previous cesarean deliveries.22,23
On the assumption of a higher average risk in our U.S. population of approximately 0.5 of 1000 births per week, we estimate that two or three stillbirths might have been averted in our cohort by delivery before 39 weeks. However, as compared with delivery at 39 weeks, earlier delivery increased the rate of adverse neonatal outcomes in the cohort, including 176 extra cases of the primary outcome, 145 admissions to the neonatal ICU, and 63 cases of respiratory distress syndrome. Although there were no neonatal deaths associated with early delivery in our cohort, the sample size was too small to evaluate an increase in this rare outcome, since more than 20,000 subjects would be needed to measure an incidence of 0.1 to 1.0% with adequate confidence and precision.
Since the vast majority of women with a previous cesarean delivery elect a repeat cesarean delivery,13,14
since more than 25% of primary cesarean deliveries are performed before the onset of labor,24
and since there may be increasing enthusiasm for cesarean delivery on maternal request,25
the timing of cesarean delivery and its effect on infant outcomes have substantial public health importance. We identified a recent systematic review26
and five individual European studies1,2,27–29
that examined the association of the timing of elective cesarean delivery with neonatal outcomes. The rates of elective cesarean delivery before 39 weeks were higher in the European cohorts (ranging from 51 to 83%) than in our study. Because these studies vary considerably in their definitions of “elective” and in their criteria for inclusion and exclusion, their results cannot be directly compared with ours. Even so, the studies consistently demonstrated increased respiratory morbidity with elective cesarean delivery before 39 weeks. These studies focused primarily on respiratory complications, and the combined number of women undergoing elective cesarean deliveries was only about half that of our study.
The composite primary outcome occurred in approximately 10% of all neonates in our study. Although the majority of deliveries before 39 weeks occurred at 38 weeks 4 days through 38 weeks 6 days, deliveries occurring during this period were also associated with increased neonatal morbidity. We also observed a higher risk of neonatal complications with cesarean delivery at 41 weeks or later, although the overall proportion of mothers delivering this late was small (<5%). These findings suggest that in addition to the risk of stillbirth, which is almost doubled at 41 weeks of gestation and increased by a factor of up to five at 42 or more weeks as compared with 39 weeks,22,23
the risk of neonatal complications may also be increased by delaying elective cesarean delivery beyond 39 or 40 completed weeks of gestation.
Our combined study population reflects the socioeconomic and demographic spectrum of the United States.30
Although we studied only repeat procedures, which constitute the majority of elective cesarean deliveries, previous studies that included primary cesarean deliveries have shown similar increases in respiratory complications with delivery before 39 weeks of gestation.28,29
Our results indicate that a high proportion of elective cesarean deliveries in the United States are performed before 39 weeks. This may be driven by several factors, including a woman’s desire to give birth once term is attained and an obstetrician’s desire to schedule the procedure at a convenient time.25
These early deliveries are associated with a preventable increase in neonatal morbidity and admissions to the neonatal ICU, which carry a high economic cost. These findings support recommendations to delay elective delivery until 39 weeks of gestation and should be helpful in counseling.