Methods
This study is a national retrospective cohort analysis using data abstracted from the National Center for Health Statistics (NCHS) and Centers for Disease Control and Prevention’s (CDC) Division of Vital Statistics database from 2015 to 2017. We chose the most recent 3 years of available data to be feasibly analyzed with our available computing hardware limits. The data is publicly available and de-identified, therefore institutional review board approval was not required.
Pregnancies with delivery <39 weeks and 0/7 days gestation or >42 weeks and 6/7 days gestation, multifetal gestation, fetal congenital anomalies, pregestational diabetes, gestational diabetes, chronic hypertension, previous cesarean delivery (CD), and infant deaths were excluded from the study cohort. Infant deaths, defined as death that occurred after 28 days of life, were excluded due to its association with postnatal complications often unrelated to the birth process.23,24 Deliveries >42 weeks and 6/7 days gestation were excluded primarily because it is no longer common practice to continue expectant management at this gestational age given the inherent neonatal morbidities associated with post-date delivery.25,26 Common maternal comorbidities were excluded to isolate the effects of elective induction of labor at 39 weeks compared with expectant management in an otherwise low-risk population. In addition to the above exclusions, the induction of labor group excluded all pregnancies diagnosed with gestational hypertensive disorders, as the American College of Obstetricians and Gynecologist (ACOG) recommends delivery for such disorders on or after 37 weeks gestation.27 As such, expectant management would not be a reasonable option in these situations. Lastly, those with spontaneous deliveries at 39 weeks of gestation who did not undergo induction of labor were also excluded.
The induction of labor group consisted of deliveries by induction of labor between 39 weeks and 0/7 days to 6/7 days gestation without an identifiable medical indication, irrespective of their final mode of delivery. The expectant management group consisted of all spontaneous deliveries from 40 weeks and 0/7 days through 42 weeks and 6/7 days gestation. Maternal demographic information was compared between the two management groups using the appropriate univariate statistical test. Statistical significance was defined as p-value <0.01.
The maternal outcomes of interest included: cesarean delivery, intra-amniotic infection or inflammation (triple I), blood transfusion, intensive care unit (ICU) admission, uterine rupture, and cesarean hysterectomy. Triple I, or chorioamnionitis as it was previously known, was identified as “clinical chorioamnionitis diagnosed during labor or maternal temperature greater than or equal to 38℃ (100.4℉).” The neonatal outcomes of interest included: 5-minute Apgar score ≤3, assisted ventilation for >6 hours, neonatal intensive care unit (NICU) admission, seizure, and neonatal death (death before 28 days of life). This data was collected through the first 28 days of life. Both maternal and neonatal data from these databases were obtained from birth certificate data. Multivariable log-binomial regression analysis was performed to control for potential confounding variables based on historic significance and univariable analysis. These variables included: maternal age, race, parity, education, prenatal care, tobacco use, and body mass index (BMI). Backward stepwise elimination method was performed to arrive at the final regression model, which included maternal age, race, education, and vaginal delivery. Power analysis was not performed as this study’s sample size included the entire population. All analyses were performed using Stata 14 statistical software (College Station, TX).28