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To examine whether time of delivery influences the risk of neonatal morbidity among women with singleton pregnancies.
Secondary analysis of data from the Maternal Fetal Medicine Units Network Factor V Leiden Mutation study. We categorized time of delivery as day (07:00–16:59), evening (17:00–23:59), and overnight (midnight–06:59). Severe neonatal morbidity was defined by at least one of the following: respiratory distress syndrome, transient tachypnea of the newborn, sepsis, seizures, neonatal intensive care admission, or a 5-minute APGAR ≤3. We calculated frequencies of severe neonatal morbidity by time of delivery. Multivariate analysis was performed to determine whether time of delivery was independently associated with severe neonatal morbidity.
Among 4,087 women, 1,917 (46.9%) delivered during the day, 1,140 (27.9%) delivered in the evening, and 1,030 (25.2%) delivered overnight. We observed no significant differences in the rates of neonatal morbidity between delivery time periods (day: 12.3%; evening: 12.8%; overnight: 12.6%; p = 0.9). No significant association was observed between time of delivery and neonatal morbidity after adjustment for maternal, obstetric, and peripartum factors.
Our findings suggest that time of delivery is not associated with severe neonatal morbidity.
Over the past 20 years, induction of labor and cesarean delivery have both become increasingly prevalent.1,2 These increases have resulted in a higher proportion of deliveries occurring during daylight hours.2,3 Although deliveries during daytime hours may be preferable to obstetric care providers, it is uncertain whether neonatal outcomes are influenced by the time of delivery.
The potential influence of time of delivery on neonatal outcomes has important clinical relevance. Variation in the rates of neonatal morbidity according to time of delivery may raise questions about the type and quality of obstetrical, anesthetic, and neonatal care provided out-of-hours. Several observational studies have investigated whether an association exists between time of delivery and neonatal morbidity, yet findings from these studies are relatively inconsistent. Several studies reported that neonatal outcomes were not influenced by time of delivery.4-7 However, the majority of these studies sourced data from single obstetric centers; thus, the generalizability of these findings is uncertain. In contrast, data from population-wide studies indicate that the risk of neonatal death is increased for deliveries in the evening or at night.8,9 Rates of neonatal morbidity were not reported in these studies. Given these findings, population-level data are needed to determine whether the risk of neonatal morbidity is increased for out-of-hours deliveries.
Using a multicenter clinical registry, the aim of this retrospective cohort study was to examine whether time of delivery influences the risk of severe neonatal morbidity. We also performed stratified analyses to determine the influence of time of delivery on neonatal morbidity according to mode of delivery and gestational age at delivery.
We performed a secondary analysis of the Eunice Kennedy Shriver National Institute of Child Health and Human Development Maternal-Fetal Medicine Units (MFMU) Network Factor V Leiden (FVL) study, which has been previously described.10 Briefly, the original study was a prospective observational study to determine the risk of pregnancy-related thromboembolism among potential carriers of the FVL mutation; no patients received anticoagulation. Study participants were pregnant women who were enrolled at no less than 14 weeks of gestational age by last menstrual period. For the original study, women were excluded if they had the following conditions: prior, current, or planned anticoagulation; previous or current thromboembolism; antiphospholipid syndrome; known FVL mutation; multiple gestation; known fetal demise; or planned termination or uncertain dates by last menstrual period. The FVL dataset was selected for our study, as the characteristics of the FVL study cohort were deemed to be generalizable to a normal delivery population with singleton pregnancies. The study was performed between April 2000 and August 2001 at 13 U.S. obstetric centers. The final FVL dataset comprised data from 4,903 pregnancies from an unselected population of patients seeking prenatal care at participating centers. The use of the FVL study data for secondary analysis was approved by the Stanford University Institutional Review Board.
For our analysis, we excluded women who had missing data for the time of delivery, mode of delivery, and gestational age at delivery. We excluded women with multiple gestations; women who delivered before 24 weeks’ gestational age; and women with pregnancies affected by a major congenital malformation, small for gestational age (SGA) (classified as less than 10th centile for birth weight), stillbirth, miscarriage, and termination of pregnancy. We categorized time of delivery into three time periods: 07:00 to 16:59 (day); 17:00 to 23:59 (evening); and midnight to 06:59 (overnight). These time periods were selected based on prior literature,4,9 and the authors’ opinions of typical times for changes of physicians’ shifts. Our primary outcome was defined by the presence of at least one of the following severe neonatal morbidities: respiratory distress syndrome, transient tachypnea of the newborn, necrotizing enterocolitis, sepsis, seizures, neonatal intensive care unit (NICU) admission, or a 5-minute APGAR score ≤3. These outcomes were selected based on literature review5-7,11,12 and clinical plausibility.
Chi-squared or Fisher exact tests were used to assess differences in maternal characteristics and rates of severe neonatal morbidity according to the time period for delivery. We calculated gestational age–adjusted frequencies for neonatal morbidity for each time period for delivery. To examine the influence of time of delivery on severe neonatal morbidity, we performed multivariate logistic regression, adjusting for the following potential confounders: maternal age, years of schooling, race/ethnicity, obesity, parity, gestational age at delivery, gestational diabetes, hypertensive disease of pregnancy, placental abruption, and mode of delivery (classified as vaginal delivery, prelabor cesarean delivery, or cesarean delivery after labor or induction). Adjusted odds ratios (aOR) and 95% confidence intervals (95% CIs) were calculated. In our secondary analyses, we performed multivariate logistic regression to investigate whether time of delivery influences neonatal outcomes stratified by mode of delivery. Statistical analyses were performed using SAS 9.3 (SAS Institute Inc., Cary, NC) and STATA version 12 (Statacorp, College Station, TX) with p < 0.05 considered as statistically significant.
For our study cohort, we excluded women with missing data for the following variables: time of delivery (n = 205), mode of delivery (n = 208), gestational age at delivery (n = 17), and SGA (n = 283). Women who delivered infants with a major congenital malformation (n = 74) and SGA (n = 420) were also excluded. We also excluded women with the following conditions: multiple gestation (n = 29); delivery before 24 weeks’ gestational age (n = 271); and women who underwent the following procedures: terminations (n = 8), ante-partum or intrapartum stillbirths (n = 58), and elective abortions (n = 36). After accounting for 846 women with at least one criterion for exclusion, our study cohort comprised 4,057 women. In our cohort, 1,905 (47%) women delivered during the day, 1,129 (27.8%) women delivered in the evening, and 1,023 (25.2%) women delivered overnight (Fig. 1). Baseline characteristics for the cohort are reported in Table 1. Women who were younger than 20 years, African American race, and other race (Asian, Native American) were more likely to deliver overnight. Compared with those who underwent prelabor cesarean delivery, women who had a vaginal delivery or cesarean delivery after labor or induction were more likely to deliver in the evening or overnight.
For our primary outcome, the unadjusted rates of severe neonatal morbidity were similar for all delivery time periods: day = 11.9%; evening = 11.6%; overnight = 12.1%; p = 0.93 (Table 2). For each individual neonatal morbidity, we observed no statistically significant between-group differences in the unadjusted rate of respiratory distress syndrome, transient tachypnea of the newborn, sepsis, seizures, NICU admission, or 5-minute APGAR score <3 (Table 2).
We calculated rates of neonatal morbidity for infants delivered at the following gestational ages: 24 to 27+6 weeks, 28 to 31+6 weeks, 32 to 36+6 weeks, 37 to 40+6 weeks, 41 to 41+6 weeks, and 42 weeks or more (Table 2). We observed no significant between-group differences in rates of severe neonatal morbidity for each gestational age category. The overall rates of severe neonatal morbidity after adjusting for these gestational age categories were 12.5% = day; 10.9% = evening, and 11.4% = overnight.
In our multivariate analyses, we adjusted for several potential confounders. Compared with daytime deliveries, the adjusted odds of severe neonatal morbidity were not significantly increased for deliveries occurring in the evening (aOR = 0.90; 95% CI = 0.7–1.2) or overnight (aOR = 1.06; 95% CI = 0.8–1.4) (Table 3). In our stratified analyses, these findings were similar for women who underwent cesarean delivery and vaginal delivery (Table 3).
In this retrospective cohort study, we sourced clinical data from a large U.S. registry of 13 different obstetric centers. We did not identify a significant association between severe neonatal morbidity and time of delivery. In our stratified analyses, time of delivery did not influence the likelihood of severe neonatal morbidity, after stratifying by gestational age at delivery or by mode of delivery.
The association between time of delivery and neonatal morbidity has been the focus of several observational studies.4-7,13,14 However, there is substantial between-study heterogeneity. Across these studies, variability existed in the definitions for neonatal morbidity, the classification of delivery time periods (“regular” hours vs. out-of-hours), and the selection of confounders in regression models. Key methodologic differences between these studies are presented in Table 4. Nonetheless, our findings are congruent with the main findings of several studies. In a study of term pregnancies at a large U.S. obstetric center, Caughey et al observed no differences in rates of neonatal morbidity or mortality between infants born to women who delivered during the daytime (7 am to 6 pm), evening (6 pm to midnight), or at night (midnight to 7 am).6 At a single center in the United Kingdom, Woodhead and Lindow observed no differences in rates of NICU admission or neonatal resuscitation of infants born between midnight and 07:59 am, 8 am, and 3:59 pm, versus 4 pm to 11.59 pm14 At a single obstetric center in Greece, Kalogiannidis et al did not observe significant differences in rates of neonatal morbidity of term babies born between 11 pm and 7 am versus 7 am to 11 pm.7 In a multicenter U.S. study, Bailit et al observed that neonatal ICU admissions were slightly increased at night for infants born by cesarean delivery between 11 pm and 7 am compared with 3 pm and 11 pm as well as 7 am and 3 pm (20.9% vs. 19.1% and 19.3%, respectively).13 However, no other differences were observed for other neonatal morbidities according to time of delivery.
It has been previously assumed that the risk of neonatal morbidity increases at night. Night-shift work is characterized by increased fatigue and decreased attention, relative to daytime work.15-17 Previous studies have shown that sleep deprivation can result in impaired psychomotor performance and clinical performance.15,17-19 Fewer in-house physicians and more limited resident supervision by attending physicians may occur out-of-hours.20 In-house or immediate availability of obstetric and anesthesia specialists during the evening and night hours may also vary between different hospitals.20 Despite these concerns, we did not find any association between neonatal morbidity and out-of-hours delivery, even after adjustment for a large set of prespecified potential confounding variables. Given that the confidence intervals for evening and overnight delivery in each model overlapped the null, it is unlikely that we have missed a major confounder that would likely or plausibly impact neonatal morbidity in a substantial fashion.
In our study, the reasons for the observed lack of association between time of delivery and neonatal morbidity are unclear. Hospitals in the MFMU network were tertiary care obstetric centers. Therefore, it is possible that many of these centers had 24 hour in-house availability of obstetricians, anesthesiologists, and neonatologists. This may have reduced variability in the quality of care and subsequent rates of neonatal morbidity over a 24-hour period.
The main strengths of our study are that we utilized prospectively collected maternal, obstetric, and neonatal data on over 4,000 pregnancies. The majority of prior studies report data from single obstetric centers. In contrast, multiple centers contributed data to the FVL dataset; therefore, our findings are more generalizable. Our study has a number of limitations. Data collection for the FVL study was performed between 2000 and 2001; therefore, our findings may be not reflective of contemporary obstetric practice. Current recommendations from the American College of Obstetricians and Gynecologists are that non-medically indicated births should not occur before 39 weeks’ gestational age.21 Therefore, future studies are needed to determine whether this change in obstetrical practice has influenced rates of neonatal morbidity. Owing to a lack of available data, we were unable to determine whether patients had an elective or nonelective cesarean delivery. Therefore, our study is prone to confounding, as elective cesarean deliveries predominantly occur during the day. Nonetheless, we were able to determine whether women underwent prelabor cesarean delivery versus cesarean delivery after a trial of labor or induction. As the majority of women who underwent prelabor cesarean delivery were delivered during the daytime period (268 women [77%]), it is possible that many of these women underwent elective cesarean delivery. Furthermore, in some cases, the risk of neonatal morbidity may be worse following elective cesarean delivery. For example, data from several studies suggest that the risk of respiratory morbidity for infants born before 39 weeks’ gestation is higher after elective cesarean delivery compared with vaginal delivery.22,23 Only a minority of patients in our cohort delivered before 37 weeks’ gestation; therefore, larger studies are needed to determine whether time of delivery influences neonatal morbidity for infants delivered preterm. As the obstetric centers in the MFMU network are tertiary care centers, it is uncertain whether these outcomes would be similar in non-tertiary community hospitals where staffing practices and availability may differ. Finally, hospitals in the FVL dataset were deidentified; therefore, hospital-specific rates of neonatal morbidity could not be ascertained.
In conclusion, we found no differences in rates of neonatal morbidity among singleton term deliveries according to the time period of delivery. Our findings may help reassure patients and providers that infants delivered out-of-hours are no more likely to incur neonatal morbidity compared with those born during daylight hours. As we sourced population-wide data from the MFMU network, future population-wide studies are needed to validate our findings and to determine whether time of delivery influences neonatal outcomes for women who deliver at nonteaching community hospitals.
We acknowledge the assistance of NICHD, the MFMU Network, and the Protocol Subcommittee in making the database available on behalf of the project. The contents of this report represent the views of the authors and do not represent the views of the Eunice Kennedy Shriver National Institute of Child Health and Human Development Maternal-Fetal Medicine Units Network or the National Institutes of Health.
This study was internally funded by the Department of Obstetrics and Gynecology and the Department of Anesthesiology, Perioperative and Pain Medicine at Stanford University School of Medicine. A. J. B. is a recipient of an award from the Eunice Kennedy Shriver National Institute of Child Health and Human Development (1K23HD070972).
Conflict of Interest
The authors report no conflict of interest.
This study was presented, in part, at the 47th Annual Society for Obstetric Anesthesia and Perinatology (SOAP) Meeting; May 13–17, 2015, Colorado Springs, CO.