Objective
To determine the trends and racial differences in early-term induction of labor in the United States.
Study Design
Data from the National Center for Health Statistics were used to identify women eligible for induction between 37-42 weeks’ gestation in the United States from 1991-2006. Annual early-term induction rates were calculated, and maternal race/ethnicity was classified into 4 groups. The change in frequency and odds of early-term induction stratified by race/ethnicity over time was assessed.
Results
Among 39.2 million eligible women, early-term induction rates increased from 2.0% to 8.0% ( P < .01) over 16 years. Cross-sectional and annual early-term induction rates were highest for non-Hispanic white women during the study period ( P < .01). After adjusting for confounding factors, the odds of any early-term induction were highest ( P < .01) and rose most rapidly ( P < .01) among non-Hispanic white women compared with women from other racial/ethnic groups.
Conclusion
In the United States, early-term induction rates rose significantly and were highest among non-Hispanic white women.
Induction of labor has become increasingly common. Although the full set of reasons for this increase is unclear, both patient preferences and changes in the provision of obstetric care have been cited as contributing factors. Established guidelines permit elective induction without medical indication after 39 0/7 weeks’ gestation; however, there is increasing evidence that the frequency of labor induction prior to 39 weeks’ has also risen. Despite this trend and a concurrent decrease in stillbirths, it remains unclear what the direct benefits are for this rise in labor induction as adverse sequelae for infants and children have been demonstrated and have been associated with increases in neonatal morbidities, infant mortality, and adverse neurodevelopmental outcomes later in childhood.
Racial differences in obstetric practice have been well documented. For example, African-American women, compared with white women, have more cesarean deliveries but receive fewer antenatal steroids, tocolytics for preterm labor, and hospitalizations for pregnancy-associated hypertension. Notably, racial differences have been shown to exist with regard to labor induction as well, with African-American women undergoing labor induction less frequently.
However, prior studies have focused on induction in general, and have not specifically evaluated the trends and racial differences in labor induction at term but before 39 weeks, a period that Engle et al have called “early-term” from 37 through 38 weeks’ gestation. As noted, early-term deliveries are of particular relevance given the increased frequency of adverse neonatal and infant outcomes in those born during this gestational age period. Thus, the aims of this study are to define the longitudinal trends in early-term (37 0/7 –38 6/7 weeks) induction (ETI) of labor and to estimate the independent and longitudinal associations between maternal race/ethnicity and ETI in the United States.
Materials and Methods
Birth certificate data from the National Center for Health Statistics (NCHS) were used to identify all women delivering in the United States from 1991 through 2006. Women who delivered a singleton between 37 0/7 and 42 6/7 weeks’ gestation were eligible to be included in the current study. Women who were not optimal candidates for labor induction were excluded from the analyses. Those with prior cesarean deliveries, fetal anomalies, with no prenatal care (as induction is generally a planned event), or with a nonvertex fetus were excluded. Also, those women whose records precluded an accurate determination of either race/ethnicity or the primary outcome measure were also excluded: those women with unknown gestational age at delivery, missing racial/ethnic designation, or unknown method of labor initiation were ineligible as well. Finally, women with pregnancy-induced hypertension (PIH) or with premature rupture of membranes (PROM) were not included in the analyses ( Figure 1 ). These last 2 groups of women were excluded given that these coded indications may develop as pregnancy progresses, and it cannot be ascertained from these data when the onset of these conditions occurred. Thus, the stability of the frequency of these conditions at 37-38 weeks over time is unknown, and the ability to adequately control for changes in these frequencies among or differences between populations, would not be possible.
The primary outcome measure was derived from the field named “induction of delivery” in the NCHS dataset. This field is defined as the “initiation of uterine contractions before the spontaneous onset of labor by medical and/or surgical means for the purpose of delivery.” Women who had labor augmentation after the spontaneous initiation of labor were not considered to have undergone labor induction.
Gestational age at delivery was based on the field designated as the “gestational age at birth” in the NCHS data set. Though some have contended that the variable coding for the “best clinical estimate of gestation” may be more valid, this variable was not collected by the state of California, which is also the state with the most annual births in each year. Thus, to optimize the sample size and the representativeness of the gravid population in the United States, the gestational age at birth variable was used. To ensure that use of the gestational age variable would not lead to biased estimates in ETI frequency, a sensitivity analysis was performed on the results using each of these 2 variables that coded for length of pregnancy and these analyses resulted in estimates that were quite similar (data not shown).
The annual ETI rate is a sum of 2 induction rates and was determined as follows: first, the number of inductions at 37 weeks’ gestation as a proportion of the number of eligible women delivering between 37-42 weeks’ gestation was calculated. Then, this rate was added to the proportion of women who were induced at 38 weeks’ as a proportion of those who remained pregnant at 38 weeks’ gestation. This sum is the annual ETI rate that is reported in these analyses (ie, Σ [# of inductions at 37 weeks/eligible women from 37-42 weeks] + [# inductions at 38 weeks/eligible women from 38-42 weeks]).
From the NCHS dataset, the study population was stratified by maternal race/ethnicity and was categorized into 4 groups: non-Hispanic white (NHW), Hispanic white (HW), Black, and Other. The “Black” and “Other” groups were not further subdivided by Hispanic ethnicity given the small proportion of women in each of these groups coded as Hispanic (3.0% and 3.3%, respectively).
Bivariable analyses were performed, using analysis of variance (ANOVA) and χ 2 analysis, to determine the demographic and medical characteristics that were significantly associated with maternal race/ethnicity. Multiple comparisons were corrected using the Bonferroni method. Demographic factors, including advanced maternal age (≥35 years old), nulliparity, smoking status during pregnancy (yes/no), alcohol use during pregnancy (yes/no), and marital status. Medical factors (maternal diabetes [DM] and chronic hypertension [CHTN]) that may increase the chance of receiving a labor induction and that were consistently measured from 1991 through 2006 were further used for stratification in the analyses.
Unadjusted longitudinal trends in ETI were examined. These trends are reported stratified by maternal race/ethnicity and by the presence of DM or CHTN, and differences in trends according to these groups were evaluated using χ 2 analyses with linear trends.
Multivariable logistic regression was used to estimate the independent association between maternal race/ethnicity and ETI (yes/no). Demographic and obstetric variables that were evaluated in the regression analysis were kept in the model if their inclusion changed the estimated odds ratio of the association between race/ethnicity and ETI by at least 10%. Maternal smoking and alcohol use during pregnancy were not evaluated in the models as potential confounding factors because of the large proportion of missing data. The models also included an interaction term between maternal race/ethnicity and the calendar year of birth to estimate whether the difference in the odds of ETI between racial/ethnic groups changed significantly over the 16-year study period.
All statistical tests were 2-tailed and α = .01 was used to define statistical significance because the large sample sizes. Analyses were performed with STATA v10.1 (STATA Inc, College Station, TX). Birth certificate data were accessed from the National Vital Statistics System website in April 2010. As these data are publicly available and deidentified, the Children’s Memorial Research Center Institutional Review Board exempted this study from review.
Results
From 1991 to 2006, 63,120,211 live born infants were delivered. Less than 1% of women had an unknown or missing racial designation, and after application of the exclusion criteria as shown in Figure 1 , 39,150,722 (62.0%) expectant mothers remained eligible for and were included in this analysis.
ETI occurred in 4.9% of eligible women and cross-sectional rates were significantly different by maternal race/ethnicity. NHW women received ETI most frequently (5.6%) and this rate differed from the rates of women from other racial/ethnic groups (HW: 3.3%, Black: 4.6%, Other: 3.6%, P < .001). Unadjusted odds ratios (OR) for the probability of ETI, according to maternal race/ethnicity, with NHW women as the reference group, are as follows: HW: OR, 0.57; 99% CI, 0.57–0.58; Black: OR, 0.79; 99% CI, 0.79–0.80; Other: OR, 0.62, 99% CI, 0.62–0.63; P < .01). Other differences among these ethnic groups are illustrated in Table 1 . Black women were more likely to be younger or unmarried; NHW women were most likely to use tobacco or alcohol; and HW women were least likely to be nulliparous.
| Characteristic | NHW | HW | Black | Other | All |
|---|---|---|---|---|---|
| n | 23,954,163 | 7,381,139 | 5,510,423 | 2,304,997 | 39,150,722 |
| Induction of labor, % | 5.6 | 3.3 | 4.6 | 3.6 | 4.9 |
| Maternal age (y ± SD) | 27.6 ± 5.9 | 25.5 ± 5.9 | 24.8 ± 6.1 | 28.5 ± 5.8 | 26.9 ± 6.0 |
| Maternal age ≥35 y | 13.0 | 8.2 | 7.8 | 15.3 | 11.5 |
| Marital status | 78.9 | 57.8 | 31.9 | 78.8 | 68.3 |
| Nulliparous | 45.4 | 41.6 | 44.0 | 48.3 | 44.7 |
| Tobacco a | 13.3 | 2.0 | 7.4 | 3.9 | 9.8 |
| Alcohol b | 1.1 | 0.3 | 1.0 | 0.6 | 0.9 |
| Chronic hypertension (CHTN) | 0.6 | 0.3 | 1.1 | 0.4 | 0.6 |
| Maternal diabetes mellitus (DM) | 2.3 | 2.4 | 2.2 | 4.2 | 2.4 |
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