Materials and Methods

This was a retrospective study based on data from the Consortium on Safe Labor (CSL), which has been described elsewhere. The study included 12 sites (including 19 hospitals) across the United States with deliveries between 2002 and 2008. Institutional review board approval was obtained from each facility within the CSL. Maternal demographic data, labor, and delivery information, birth records, and neonatal information were extracted from the patient electronic medical records and supplemented with discharge summary codes. Validation studies comparing discharge codes with medical chart review confirmed high level of accuracy. Overall, there was 91.1% to 95% concordance with the medical chart for 20 variables examined.

Data was also collected from chart review for all infants delivered >37 weeks’ gestation who required respiratory support in the delivery room with oxygen or ventilation and were admitted to the neonatal intensive care unit (NICU) with a respiratory diagnosis. Gestational age at delivery as recorded in the medical record was based on the best obstetric estimate available.

There were 228,438 deliveries from 208,695 women in the CSL. We limited the current analysis to 188,809 singleton deliveries from 173,569 women with gestational ages between 37 0/7 to 41 6/7 weeks. Precursors for delivery were categorized as previously described into 4 exclusive categories: spontaneous labor, premature rupture of membranes (PROM), indicated, and no recorded indication. We classified the precursors for delivery according to the following hierarchy: women who presented in spontaneous labor with and without pregnancy complications or rupture of membranes were only included in the spontaneous labor category. A pregnancy was determined to be PROM exclusive of spontaneous labor if a woman presented with premature rupture of membranes and underwent an induction of labor or prelabor cesarean delivery. Indicated deliveries included women without spontaneous labor or PROM who underwent an induction of labor or prelabor cesarean delivery for maternal, fetal, or obstetric complications of pregnancy. If an induction of labor or indication for prelabor cesarean delivery was not recorded, but the pregnancy had other complications such as hypertensive disease or diabetes, it was also categorized as indicated. Therefore, a woman could potentially have multiple pregnancy conditions in the indicated category. Finally, admission for a fetal or maternal reason not specified in the above categories, as well as history of fetal or maternal condition not present in the current pregnancy were considered indicated. Postdates, defined as 41 0/7 to 41 6/7 weeks of gestation, was also counted as an indicated delivery exclusive of other indications. In a previous study, we determined that birth outcomes for deliveries that had “elective” stated as the indication were similar to that for uncomplicated pregnancies with no recorded indication. Therefore, we combined elective cesarean deliveries and labor inductions with no other complications noted in the medical record along with deliveries without a recorded indication into the no recorded indication category.

The incidences of neonatal outcomes for the specific delivery precursors by gestational week were calculated. Neonatal outcomes included neonatal intensive care unit (NICU) admission; respiratory morbidity; sepsis or evaluation to rule out sepsis; hypoxic ischemic encephalopathy, asphyxia, or seizures; and birth trauma. Neonatal respiratory morbidity included any of the following conditions requiring NICU admission: oxygen therapy with nasal cannula, continuous positive airway pressure, bilevel positive airway pressure, ventilator use, or diagnosis of respiratory distress syndrome, hyaline membrane disease, pneumonia, apnea, bradycardia, pulmonary hypertension, pneumothorax, meconium aspiration, or pulmonary hypoplasia. Birth trauma included epicranial subaponeurotic hemorrhage, scalp injury, clavicular fracture, facial nerve or brachial plexus injury, other cranial or peripheral nerves injuries, and other specified or unspecified birth trauma. Composite neonatal morbidity, which included NICU admission, respiratory morbidity, sepsis or evaluation to rule out sepsis, neurologic morbidity, and birth trauma was calculated at each week gestational age for each major precursor. Because cesarean delivery is an independent risk factor for respiratory morbidity, we compared estimates of planned and actual route of delivery on neonatal respiratory morbidity by gestational age at delivery and precursor.

Characteristics of mothers and newborns according to precursor for delivery were calculated for early term (37-38 6/7 weeks) and full term (39-41 6/7 weeks) birth. Significance testing was performed to compare characteristics across the 4 major precursors of early term birth using either ordinal (categoric variables) or logistic (binary variables) regression with generalized estimating equations (GEE) to account for correlations between pregnancies to the same mother. The rates of deliveries for each gestational week were calculated according to precursor with significance testing performed using generalized linear models with GEE. Deliveries with anomalies (n = 11,559) and antepartum stillbirth before onset of labor (n = 228) were then excluded. There were 7 deliveries associated with both anomalies and stillbirth leaving 177,029 deliveries in our analyses of morbidities. The neonatal morbidity rates for each precursor were calculated for each gestational week at delivery and tested for significance using generalized linear models with GEE to obtain a global P value with contrast statements to calculate for significant differences between individual weeks of gestation (eg, 37 vs 39 and 38 vs 39 weeks). A similar analysis was performed to calculate rates of serious respiratory morbidity by gestational age at delivery and precursor according to planned and actual route of delivery. The incidences of stillbirth or perinatal mortality in ongoing pregnancies reaching a specific week of gestation were compared using Cox proportional hazard models. Given the number of comparisons, a P value of < .01 was considered significant. Statistical analysis was performed using SAS 9.3 (SAS Institute, Inc., Cary, NC).

Results

Among term deliveries, 64,476 (34.1%) were early term between 37 0/7 and 38 6/7 weeks’ gestation although 124,333 (65.9%) were full term between 39 0/7 and 41 6/7 weeks. Characteristics of mothers and newborns according to precursor for delivery are presented in Table 1 . Women who underwent an indicated delivery during the early term period were more likely to be nonHispanic black, have a history of preterm birth, and have an infant weighing <2500 or >4000 g compared with deliveries among the other 3 precursors for early term birth ( P < .001).

Overall, 53.6% of term deliveries occurred with spontaneous labor, followed by indicated (27.6%), no recorded indication (15.5%), and PROM (3.3%) ( Table 2 ). With increasing gestational age at delivery until 39 weeks, the percentage of spontaneous labor, PROM, and most indications decreased although those with no recorded indication increased. At 40 weeks of gestation the spontaneous labor rate was highest (58.1%, P < .001).

We calculated composite neonatal morbidity across gestational age for each major precursor ( Figure 1 , A). Composite neonatal morbidity for spontaneous labor, indicated, and no known indication decreased for neonates delivered at 39 weeks compared with 37 weeks ( P < .001). There was no difference in composite morbidity for PROM deliveries across term gestation ( P = .162). Furthermore, there was no significant decrease in composite neonatal morbidity between 38 and 39 weeks for any major delivery precursor ( P > .01 for all comparisons). The majority of neonatal morbidities were lowest at 39 weeks’ gestation ( Figure 1 , A). We therefore used 39 week’s gestation as our reference group for statistical comparison.

Neonatal morbidities by gestational age and precursor

HIE , hypoxic ischemic encephalopathy; PROM , premature rupture of membranes.

^a P values for legend correspond to composite morbidity across all weeks gestational age for each major delivery precursor. P value < .001 for deliveries at 37 compared with 39 weeks for all major precursors except for PROM ( P = .016). P values nonsignificant for 38 vs 39 weeks for each major delivery precursor ( P = .029–.545); ^b P values for legends in B-D correspond to morbidity across all weeks gestational age for each major delivery precursor.

Parikh. Early term neonatal morbidity. Am J Obstet Gyncol 2014 .

NICU admission rates were higher for deliveries at 37 compared with 39 weeks for spontaneous labor, PROM, indicated, and no recorded indication ( P < .001 for all), but were not different for delivery at 38 vs 39 weeks except for indicated deliveries (7.5% vs 6.6%, P = .003) ( Table 3 ). There was a 35% to 54% decrease in NICU admission from 37 to 39 weeks depending on the precursor with the largest absolute decrease occurring between weeks 37 to 38 for most precursors. NICU admission rates were not significantly different for deliveries at 38 vs 39 weeks except for pregnancies complicated by gestational diabetes (9.7% vs 7.1%, P = .008). Even in the low-risk group of deliveries with no recorded indication, NICU admission rates were increased for deliveries at 37 compared with 39 weeks (7.2% vs 3.3%, P < .001), but not significantly different for deliveries at 38 vs 39 weeks (4.0% vs 3.3%, P = .035) ( Table 3 ).

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