Objective
We sought to quantify the gestational age–specific morbidity of black vs white neonates.
Study Design
This was a population-based retrospective cohort study of singleton live births in Ohio from 2006 through 2007. The primary outcome was a composite of adverse neonatal outcomes of ≥1 morbidity: Apgar score <7 at 5 minutes, assisted ventilation >6 hours, seizures, or neonatal transport to a tertiary care facility. Generalized linear regression estimated the relative risk of adverse neonatal outcome by week of gestation after adjustment for influential coexistent risk factors.
Results
The frequency distribution curve of composite morbidity by gestational age were similar, but shifted to left (earlier gestational age) for black compared with white neonates. Adverse outcome was lower for black compared with white births at each preterm week of gestational age. The lowest adverse outcome rate for black neonates was at 38 weeks and 39 weeks for white neonates, each increasing by week of gestation thereafter.
Conclusion
These data suggest that pregnancies in black women perhaps have a shorter physiologic gestational length.
Prior studies have demonstrated a higher frequency of preterm birth and lower frequency of postterm birth in black compared with white women. Additionally, gestational age–specific neonatal morbidity is lower in black compared with white births at preterm gestational ages. These prior studies suggest that the “normal” gestational length of black women may be shorter than that of white women. The aim of this study was to quantitatively assess the gestational age–specific difference in neonatal morbidity of black vs white infants.
Materials and Methods
This study was approved by the Ohio Department of Health and Human Subjects Institutional Review Board and a deidentified dataset was provided by the Child Policy Research Center of Cincinnati Children’s Hospital Medical Center. The study was exempt from review by the Institutional Review Board at the University of Cincinnati, Cincinnati, OH.
We conducted a population-based retrospective cohort study using the Ohio Department of Health’s birth certificate database (2006 through 2007) to assess racial differences in the frequency of adverse neonatal outcomes, stratified by week of gestation. We examined birth records of all singleton live births excluding major congenital anomalies, births <20 or >44 weeks, births to mothers with any self-reported race other than white or black, missing race, and mothers listing race as both black and white ( Figure 1 ). After exclusions, 273,256 births remained for analysis.
The primary outcome was a composite of adverse neonatal outcomes consisting of ≥1 of the following: Apgar score <7 at 5 minutes, assisted ventilation >6 hours, neonatal seizures, or neonatal transport to a tertiary care facility. We focused our assessment of neonatal outcomes to those occurring in the immediate (24-48 hours) postdelivery period as those occurring later would not be captured at the time of birth certificate generation. The frequency and risk of adverse outcome was compared between black and white newborns by week of gestational age, from 32-41 weeks. Less than 1% of deliveries occurred >41 weeks, thus analysis was limited to these earlier gestational ages. At gestational ages <32 weeks the high rate of morbidities present regardless of race, as well as far fewer births at these early preterm gestational ages, limited our ability to make accurate week-by-week outcome comparisons at early gestational ages. As such the analysis was limited to 32-41 weeks of gestation. The variable “combined EGA,” which is an estimate of gestational age using a combination of last menstrual period, ultrasound, and clinical dating–as commonly used in clinical practice–was used for this analysis.
There were minimal missing data for the exposure variables and primary outcomes of interest in this study. The dataset included complete data on maternal race in 307,496 (99.7%) and gestational age in 307,880 (99.8%) births. Maternal race information was missing in only 815 (0.3%) birth records, and these were excluded from analysis. Data on father’s race were missing in 26,987 (11.8%) of white mothers and 22,298 (49.2%) of black mothers. For the purposes of this study, the neonate’s race was considered the same as the mother’s race. As the reported identity and race of the father on the birth record is subject to error, misreport, or missing data, it was not considered as part of the definition of neonatal race in this study. There were minimal missing data (<5%) for each of the covariates included in the adjusted analysis, except for maternal weight, which had 7.9% missing data.
Statistical analyses were performed using software (STATA, release 10; StataCorp, College Station, TX; and SAS, version 9.2; SAS Institute Inc, Cary, NC). Demographic characteristics were compared using the unpaired Student t test for continuous variables, and χ 2 or Fisher exact test as appropriate for categorical variables. We utilized generalized linear regression to estimate adjusted relative risks (aRR) for the influence of race on composite adverse neonatal outcome stratified by week of gestation. Risk estimates were adjusted for statistically significant and biologically plausible coexisting risk factors including maternal hypertension, diabetes, limited prenatal care, hospital level, neonatal sex, and tobacco use. As there is no clearly defined standard metric for quantity of prenatal care, we defined limited prenatal care as <5 prenatal visits for the purposes of this study. Adjustment for birthweight did not change the interpretation of the results, but in some models caused collinearity between covariates. For this reason, birthweight was not included in the final adjusted model. The aRR and 95% confidence intervals (CIs) were reported for each of the primary outcomes. Comparisons with a P value of < .05 or 95% CI not inclusive of the null were considered significant.
Results
There were 308,380 births in Ohio during the study period. After initial exclusions, 293,877 births remained ( Figure 1 ). Of those births, 227,910 (77.6) were to white mothers; 45,346 (15.4%), black; 1997 (0.7%), biracial (race reported as both black and white); and 18,624 (6.3%) were births to mothers of other race. This analysis was limited to births to black and white mothers only, n = 273,256.
Demographic differences of the population are outlined in Table 1 . Black mothers were younger but had a higher parity compared with white mothers. Black mothers were more likely to be obese, have a prior preterm birth, or have a preexisting medical complication of hypertension or diabetes. Black mothers were more likely to have a limited number of prenatal care visits, be unmarried, and have state Medicaid insurance. The reported race of the father of the baby was the same as the mother in 96.4% of white and 93.1% of black mothers, although data on paternal race were missing for 11.8% of white mothers and 49.2% of black mothers. Mode of delivery was similar between black and white parturients.
