Objective
The objective of the study was to determine the trends and racial differences in late preterm induction (LPI) of labor in the United States.
Study Design
Data from the National Vital Statistics System were used to identify women eligible for induction between 34 and 42 weeks’ gestation from 1991 to 2006. Annual LPI rates were calculated, and maternal race/ethnicity was classified into 4 groups. Changes in the frequency and odds of LPI, stratified by race/ethnicity, were assessed using logistic regression.
Results
Among the 42.0 million eligible women, LPI rates increased from 0.46% to 1.37% ( P < .01) over 16 years. LPI rates were highest for black women ( P < .01) each year, and after adjusting for confounding factors, the odds of LPI were highest ( P < .01) and rose most rapidly ( P < .01) for black women (non-Hispanic white: odds ratio [OR], 1 [referent]; Hispanic white: OR, 0.76; black: OR, 1.31; other: OR, 0.81; P < .01).
Conclusion
LPI rates were persistently highest and rose most rapidly for black women.
Davidoff et al have demonstrated that, over a recent 13 year period, the mean gestational age at birth among singletons decreased by 1 week in the United States. This decrease has been due not only to women delivering earlier at term gestations but also delivering more frequently at preterm gestations, most notably during the late preterm period (34 0/7 to 36 6/7 weeks’ gestations). Moreover, late preterm delivery (LPD) has been rising despite a growing body of literature that highlights the adverse short- and long-term outcomes associated with LPD.
For Editors’ Commentary, see Table of Contents
An increasing rate of induction of labor among women with term gestations has also been well documented during the same time period. Provider practices, patient preferences, and risk reduction for stillbirth are reasons that have been suggested to have contributed to this rise in term labor induction; however, the specific reasons that have contributed to the rise in preterm induction, and the resulting LPD rates remain largely unmeasured and uncertain.
Recent evidence suggests that nonmedical factors may be associated with the increase in these inductions, and the existence of a racial/ethnic disparity of labor induction supports the concept of nonmedical contributing factors. For example, non-Hispanic white (NHW) women have been most likely to be induced at term gestation but prior to 39 weeks despite the fact that they are less likely to have medical comorbidities.
However, the extent to which there is a racial/ethnic disparity among women induced at preterm gestations remains unknown. Thus, the aims of this study were to: (1) determine national trends of late preterm induction of labor (LPI) and (2) to estimate the independent association between maternal race/ethnicity and LPI during a recent 16 year period in the United States.
Materials and Methods
Birth certificate data from the National Vital Statistics System (NVSS) were used to identify all women delivering in the United States from 1991 through 2006. Women who delivered a singleton between 34 0/7 and 42 6/7 weeks’ gestation were included in the analysis. As shown in Figure 1 , women with prior cesarean deliveries or with a nonvertex fetal presentation were not included because these circumstances often have been considered to be contraindications to labor induction. Those with fetal anomalies or no prenatal care (because induction of labor is generally planned during antenatal care) were excluded as well.
Also omitted, were those women whose records precluded an accurate determination of either race/ethnicity, gestational age at delivery, or whether labor induction occurred, given that the selected exposure (race/ethnicity) or primary outcome (LPI) could not be determined without these data. Lastly, women coded as having pregnancy-induced hypertension (PIH) or with premature rupture of membranes (pROM) were not considered in these analyses. The timing of onset or the severity of these conditions was unknown, which precluded the ability to control for the presence/severity of either PIH or pROM within the late preterm period. Thus, women with either PIH or pROM were all excluded to reduce the possibility of confounding bias.
LPD was defined as delivery of a live infant between 34 0/7 and 36 6/7 weeks’ gestation. LPI was defined as induction of labor within this late preterm period. Induced women were identified in the NVSS dataset as women had “induction of labor,” defined in the dataset as “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. Women who had LPD via a cesarean delivery were not considered because the presence or onset of labor prior to their delivery could not be ascertained.
Gestational age was determined from the variable of gestational age at birth. Although some have suggested that the best clinical estimate of gestation may be a more valid variable, this variable was not collected by the state of California, which is also the state with the most annual births. Thus, to maximize the sample size and the representativeness of the national gravid population, the gestational age variable was used in the primary analysis. To ensure that the selection of the gestational age variable did not significantly affect the results, an additional sensitivity analysis that utilized the alternative variable of best clinical estimate of gestation was performed.
LPI rates were calculated as the sum of 3 week-specific induction rates within the late preterm period (ie, Σ [inductions at 34 weeks/eligible women from 34 to 42 weeks] + [inductions at 35 weeks/eligible women from 35 to 42 weeks] + [inductions at 36 weeks/eligible women from 36 to 42 weeks]). Annual and race-stratified rates of LPI are reported.
Race/ethnicity was categorized into 4 groups: NHW, Hispanic white (HW), black, and other. The black and other groups were not further subdivided by Hispanic ethnicity, given the small proportion of Hispanic women in each of these groups (3.0% and 3.3%, respectively).
Bivariable analyses were performed, using analysis of variance (ANOVA) and χ 2 to determine the demographic and medical characteristics that were significantly associated with maternal race/ethnicity. Demographic factors included advanced maternal age (≥35 years old), teen pregnancy (age 13-19 y), nulliparity (yes/no), smoking status during pregnancy (yes/no), alcohol use during pregnancy (yes/no), and marital status (yes/no). The data were further stratified by medical factors (maternal diabetes mellitus [DM] or chronic hypertension [CHTN]) that are associated with labor induction and that were consistently measured from 1991 through 2006 in the birth certificate registry.
Multivariable logistic regression was used to estimate the independent association between maternal race/ethnicity and LPI (yes/no). Demographic and obstetric variables, evaluated in the regression analysis, were kept in the model if inclusion changed the estimated odds ratio of the association between race/ethnicity and LPI by at least 10%. Maternal smoking and alcohol use during pregnancy were not included in the models as potential confounding factors because of the large amount 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 LPI by racial/ethnic group changed significantly over the 16 year study period.
All statistical tests were 2 tailed and, because of the large sample size, α = 0.01 was used to define statistical significance. Analyses were performed with STATA version 10.1 (StataCorp, College Station, TX). Birth certificate data were accessed from the National Vital Statistics System web site in April 2010. Because these data are publicly available and deidentified, the Children’s Memorial Research Center Institutional Review Board exempted this study from review.
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