Study population

We selected our study cohort from the database of the Consortium on Safe Labor, a study conducted by the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD), National Institutes of Health. In brief, this was a retrospective cross-sectional study involving deliveries from 2002 through 2008 from 12 clinical centers and 19 hospitals representing 9 American Congress of Obstetricians and Gynecologists (ACOG) districts. The population was then standardized by assigning a weight to each subject using ACOG district, maternal race/ethnicity, parity, and plurality based on 2004 national data. Institutional review board approval was obtained for this analysis.

The Figure presents the flow diagram for our cohort selection. From the initial data set of all nulliparous and multiparous women presenting for delivery, we started with the first singleton pregnancy for each woman (233,736). We then limited the group to term gestations of 37-42 weeks’ gestational age (178,575) and in vertex presentation (155,848). To limit confounding, we excluded all those with a prior uterine scar or planned (elective) cesarean delivery (136,014). Finally, we excluded those with chronic maternal conditions that may lead to indicated delivery, including diabetes mellitus, chronic hypertension, cardiovascular disease, placental previa, or human immunodeficiency virus–positive status. Women who developed a pregnancy-related complication such as preeclampsia, gestational hypertension, abruption, or fetal compromise were included in the final cohort of 131,243 low-risk women. If these complications developed antepartum, the patients were no longer a candidate for an elective induction of labor, and they were included only in the expectant management arm of the study. We then identified women with an elective induction of labor. A predefined variable for induction of labor was coded when either the patient’s electronic medical record indicated that there was an induction and/or a method or start time for induction. This variable did not include women receiving only labor augmentation or those without at least 2 vaginal examinations in the labor progression database. Inductions were categorized as “indicated,” “elective,” and “no recorded indication.” Indications for induction included all potential maternal, fetal, or obstetric complications of pregnancy. If a site indicated that the induction was elective, no other indications for induction were provided, and there were no other obstetric, fetal, or maternal conditions complicating the pregnancy, then the induction was designated “elective.” All other deliveries, including the “no recorded indication” category were included in the expectant management group. Method of labor induction included all methods for cervical ripening and induction with multiple methods was allowed.

Flow diagram of cohort selection and distribution of subjects by week of pregnancy

The selection of our low-risk term cohort and the distribution of our final cohort into those electively induced or expectantly managed by week of pregnancy.

HIV , human immunodeficiency virus.

Gibson. Outcomes in electively induced low-risk term pregnancies. Am J Obstet Gynecol 2014 .

Study outcomes

Subjects were divided by week of gestational age at delivery ( Figure ). Those with an induction of labor coded as “elective” were the cases at each week of gestation (elective induction of labor). Those not electively induced and who delivered after that week of gestation were considered to be expectantly managed in that week of gestation. For example, the women who delivered in their 37th week of gestation after an elective induction were compared to those who delivered in their ≥38th week as they had been expectantly managed in their 37th week. Mode of delivery was categorized as a nonoperative vaginal delivery, an operative vaginal delivery (requiring vacuum or forceps assistance), or cesarean section.

Given the limitations of pregnancy dating and the size of our data set, we chose to evaluate gestational age by week rather than day. We acknowledge that many women who are expectantly managed may have either a spontaneous labor or indicated induction in the same week. Therefore, we did a secondary analysis comparing those with elective induction of labor to those expectantly managed, but ultimately delivering within the same week.

The primary outcome of interest was mode of delivery. Secondary outcomes included composites for maternal and neonatal comorbidities. Maternal outcomes included bleeding (blood products, abruption, postpartum hemorrhage, uterine rupture, hysterectomy), maternal ICU admission or death, infections (intrapartum fevers, chorioamnionitis, endomyometritis, wound separation), lacerations (third- or fourth-degree perineal, sulcal, or cervical), and shoulder dystocia. Neonatal outcomes included a composite of major comorbidities (birth injuries, sepsis, pneumonia, intraventricular hemorrhage, aspiration, hypoxic ischemic encephalopathy, respiratory distress syndrome, seizures, oliguria, myocardial injury, ventilator use, continuous positive airway pressure use, transient tachypnea of the newborn, transfusions, or surfactant use), a composite of respiratory morbidities (oxygen use, continuous positive airway pressure use, transient tachypnea of the newborn, or surfactant administration), or perinatal death (intrauterine fetal demise or neonatal demise).

To adjust for possible confounding factors on successful induction of labor and maternal and neonatal outcomes, logistic regression was performed. We controlled for maternal age, race/ethnicity, parity, body mass index at delivery, insurance status, type of hospital, and modified Bishop score at delivery admission. As only 51% (67,257) of patients had all 5 components of the Bishop score reported, while 84% (110,432) had information on cervical dilation, effacement, and station available, we used a modified Bishop score to determine cervical ripeness. We defined an unripe cervix as a modified Bishop score ≤4.

Statistical analysis

Given that this is a descriptive analysis with very large sample size, power calculations were not performed. The χ ² and t tests were used where appropriate. Demographic factors were controlled for with logistic regression. Centers with >5% missing data for an outcome were removed from the analysis on that specific outcome. We performed all analyses using statistical software (SAS, version 9.3; SAS Institute Inc, Cary, NC).

Patients

Our low-risk patient population included 131,243 women. Table 1 presents the maternal demographic characteristics for the entire cohort and by maternal parity. Our patients were nearly half nulliparous with 88% being younger than 35 years at delivery. The majority were Caucasian (51.7%) and overweight or obese (mean body mass index: 30.3 ± 5.8 kg/m ² ). Only 1.2% of patients were self-paying with >90% delivering at a teaching hospital (40.2% university and 50.8% community teaching hospitals). In all, 57% had a favorable cervix on admission. On average, subjects delivered at 39.3 ± 1.1 weeks’ gestational age.

The distribution of elective deliveries by week of gestation and parity is also presented in Table 1 . Ten percent of the total cohort (n = 13,242) had an elective induction of labor with 57% of all elective inductions performed in the 39th week of gestation. The majority of elective inductions were parous (68%), particularly at 37, 38, and 39 weeks of gestation. The Figure shows the number of electively induced deliveries, nonelectively induced deliveries and expectantly managed subjects for each week of gestation.

Study outcomes

Table 2 presents the findings of the primary outcome, mode of delivery. Nonoperative vaginal delivery occurred more often in the electively induced group when compared to the expectantly managed group at each week of gestation. Additionally, the frequency of both operative vaginal delivery and cesarean delivery was significantly lower for the electively induced group at each gestational age in both nulliparous and multiparous patients with either a favorable or unfavorable cervical status on admission. The odds of a cesarean delivery after an elective induction of labor vs expectant management were lower at each week of gestation after controlling for possible confounding factors in a logistic regression. (Adjusted odds ratio for births at 37 weeks in nulliparous patients with an unfavorable cervix = 0.40 [95% CI, 0.18–0.88]; 38 weeks = 0.65 [95% CI, 0.49–0.85]; 39 weeks = 0.47 [95% CI, 0.38–0.57]; 40 weeks = 0.69 [95% CI, 0.59–0.81]; in multiparous patients with a favorable cervix at: 38 weeks = 0.42 [95% CI, 0.16–1.14]; 39 weeks = 0.35 [95% CI, 0.23–0.53]; 40 weeks = 0.54 [95% CI, 0.30–0.97].) The comparison of the primary outcome for those electively induced compared to those who delivered within the same week (without elective induction) demonstrated similar findings ( Supplemental Table 1 ).

As the decision to deliver a patient, or allow a pregnancy to continue until indicated delivery or spontaneous labor occurs, encompasses many potential outcomes for the mother and neonate, we next evaluated several secondary maternal and neonatal outcomes ( Tables 3 and 4 ). The risk of maternal bleeding complications, ICU admissions, or death was overall similar between the 2 groups after adjustment for confounding factors. Elective induction of labor appeared to be protective against maternal infection at each week of gestation for both nulliparous and multiparous patients (nulliparous patients at: 37 weeks = adjusted odds ratio, 0.35 [95% CI, 0.13–0.95]; 38 weeks = 0.35 [95% CI, 0.23–0.53]; 39 weeks = 0.41 [95% CI, 0.33–0.50]; 40 weeks = 0.45 [95% CI, 0.38–0.55]; in multiparous patients at: 37 weeks = 0.21 [95% CI, 0.03–1.54]; 38 weeks = 0.20 [95% CI, 0.08–0.48]; 39 weeks = 0.34 [95% CI, 0.25–0.47]; 40 weeks = 0.72 [95% CI, 0.47–1.08]). The frequency of a significant obstetrical laceration was lower with elective induction in nulliparous patients at 37 and 38 weeks and in multiparous patients at 39 and 40 weeks, but only significant for the multiparous patients after adjustment. Elective induction was protective against shoulder dystocia at 38 weeks of gestation for multiparous patients. These results are similar to the findings for women electively induced compared to those delivering in the same week ( Supplemental Table 2 ).

Table 3

Secondary outcomes of maternal morbidity by week of elective induction of labor compared to those expectantly managed divided by parity

Nulliparous	Maternal bleeding complications
Wk	eIOL		Exp		aOR	95% CI
	No.	%	No.	%
37	1/65	1.5	2990/42,230	7.1	0.211	0.029–1.521
38	12/368	3.3	2357/33,215	7.1	0.414	0.232–0.740
39	119/1401	8.5	1300/18,535	7.0	1.010	0.827–1.233
40	127/1569	8.1	317/5095	6.2	0.987	0.784–1.243

Wk	Maternal intensive care admission or death
	eIOL		Exp		aOR	95% CI
	No.	%	No.	%
37	0/72	0.0	97/44,728	0.2	–	–
38	0/399	0.0	69/35,117	0.2	–	–
39	0/1.566	0.0	45/19,600	0.2	–	–
40	4/2060	0.2	11/5124	0.2	0.984	0.273–3.552

Wk	Maternal infections
	eIOL		Exp		aOR	95% CI
	No.	%	No.	%
37	4/68	5.9	7155/46,978	15.2	0.345	0.125–0.952
38	23/382	6.0	5886/37,109	15.9	0.347	0.227–0.530
39	97/1540	6.3	3591/20,844	17.2	0.405	0.328–0.500
40	152/1881	8.1	1092/5600	19.5	0.454	0.377–0.546

Wk	Third- or fourth-degree perineal or sulcal lacerations
	eIOL		Exp		aOR	95% CI
	No.	%	No.	%
37	5/72	6.9	6206/58,783	10.6	0.767	0.308–1.910
38	35/404	8.7	4997/46,570	10.7	0.969	0.683–1.377
39	173/1576	11.0	2902/26,605	10.9	0.896	0.759–1.057
40	235/2124	11.1	790/7447	10.6	0.978	0.829–1.153

Wk	Shoulder dystocia
	eIOL		Exp		aOR	95% CI
	No.	%	No.	%
37	0/72	0.0	697/56,479	1.2	–	–
38	2/404	0.5	580/44,671	1.3	0.357	0.088–1.439
39	27/1576	1.7	341/25,402	1.3	1.292	0.865–1.929
40	29/2124	1.4	95/6997	1.4	1.134	0.732–1.757

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