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