GRAPEVINE, TEX. – College education and high socioeconomic status do not erase U.S. racial disparities in birth outcomes, according to a new analysis of all U.S. live births from 2015-2017.
Very early preterm birth – birth before 28 weeks gestational age – was five times more likely to occur in non-Hispanic black women of high socioeconomic status as similarly situated white women, even after statistical adjustment for a host of potentially confounding factors.
Being of non-Hispanic black race was the single strongest predictor of preterm birth (PTB) at less than 28 weeks’ gestation. The adjusted odds ratio (aOR) of 4.99 surpassed an interpregnancy interval under 1 year (aOR, 4.47), chronic hypertension (aOR, 2.84), and prior history of preterm birth (aOR, 2.81).
“Even among college-educated women with private insurance who are not receiving (support), racial disparities in prematurity persist. These results suggest that factors other than sociodemographics are important in the underlying pathogenesis of PTB and in etiologies of racial disparities,” wrote , and her coauthors in the abstract accompanying the presentation at the meeting sponsored by the Society for Maternal-Fetal Medicine.
The analysis that Dr. Johnson and her coinvestigators used, she explained during her plenary session presentation, still found significantly elevated risks for preterm birth for non-Hispanic black women after accounting for marital status, prior history of preterm birth, tobacco use, an interpregnancy interval of fewer than 12 months, and carrying a male fetus.
“Birth certificates do not inform the lived experiences of one’s self-identified race, and how that experience – or possibly just one’s identification with a particular racial group – may positively or negatively affect their clinical risk of preterm birth,” said Dr. Johnson. “In this study, as in others, race is a social construct. It’s a surrogate for social and societal racism that disproportionately affects birth outcomes of women of color.”
Using non-Hispanic white (NHW) women as a reference, women who described themselves as non-Hispanic black (NHB) had increased odds of preterm birth at 34 and 37 weeks gestation as well. Women identifying as both NHB and NHW had numerically elevated odds for preterm birth at all time points as well, but the odds at 37 weeks didn’t reach statistical significance.
The results were based on a retrospective population-based study of a cohort drawn from the National Vital Statistics birth certificate data of all live births in the United States between 2015 and 2017, explained Dr. Johnson, a maternal-fetal medicine fellow at the University of North Carolina, Chapel Hill. Drawing from a nationally representative sample and having a population-level design drawn were strengths of the study, she said.
Women with singleton pregnancies without anomalies who identified as NHB, NHW, or as both NHB and NHW were included if they also had high socioeconomic status. Including women who identify as both black and white was another strength of the study, Dr. Johnson added.
She explained that, for the purposes of the study, high socioeconomic status was defined as having 16 or more years of education and private insurance, and not receiving WIC benefits.
In addition to the primary outcome measure of preterm birth at fewer than 37 weeks gestation, secondary outcomes included preterm birth at fewer than 34 and fewer than 28 weeks’ gestation, as well as low birthweight (LBW) and very low birthweight (VLBW).
About 11.8 million live births occurred during the study period, and 11.3 million of those were singleton pregnancies without fetal anomaly. After excluding women who did not meet the racial self-identification or socioeconomic status inclusion criteria, the investigators arrived at the final study population of 2,170,688 individuals.
Of those, 2,017,470, or 92.9%, were non-Hispanic white, while 144,612, or 6.7%, were non-Hispanic black. The remaining 8,604 participants, or 0.4%, identified their race as non-Hispanic black and non-Hispanic white.
The groups identified in the study differed significantly in demographic characteristics, Dr. Johnson said. Women in the NHB and NHB + NHW groups were less likely to be married than NHW women – about 75% of the former two groups were married, compared with 92.5% of NHW women. This difference was statistically significant with a P value of .001, as were all the differences Dr. Johnson reported.
Pre-pregnancy body mass index (BMI) was highest in NHB women at 27.1 kg/m sq, followed by NHB = NHW women at 25.7 kg/m sq, with NHW women having the lowest BMI at 23.8 kg/m sq.
Prior preterm birth of 37 weeks’ gestation or less was more common in NHB women and NHB + NHW women, as was an interpregnancy interval of fewer than 12 months.
Chronic hypertension was more than twice as common in NHB women than in either NHB = NHW or NHW women, occurring in 3.9%, 1.8%, and 1.4% of participants, respectively.
Pregestational diabetes was about twice as common in NHB women than NHW women, occurring in 1% and 0.52% of those groups, respectively. Prevalence of pregestational diabetes was intermediate in NHB = NHW women, at 0.72%.
Tobacco use was rare overall, and less common in NHB women than NHB + NHW and NHW women.
In terms of pregnancy characteristics, though 85% of NHB women initiated prenatal care in the first trimester, they were less likely to have done so than either of the other two groups. Few women overall had no prenatal care, but 0.7% of NHB women fell into this category, more than the 0.4% and 0.3% reported for NHB + NHW and NHW women, respectively.
During their pregnancies, NHB women were more likely to develop gestational hypertension and/or pre-eclampsia as well as gestational diabetes than either of the other two groups (7.6% compared with 6% for the other two groups). Of the NHB women, 5.9% developed gestational diabetes, compared with 4.8% of NHB + NHW and 4.8% of NHW women.
Delivering a baby with a birthweight less than the 10th percentile was twice as common for NHB, compared with NHW women (7.2% versus 3.4%). The risk for NHB + NHW women was intermediate, at 4.7%.
Dr. Johnson said she and her team performed further analyses, including using initiation of prenatal care in the first trimester of pregnancy as a surrogate for high socioeconomic status; the association of race and risk for preterm birth persisted.
The study had the usual limitations of using National Vital Statistics data, such as the inability to evaluate underlying etiologies for preterm birth.
However, Dr. Johnson highlighted additional limitations that pertain to the experience of race in 21st century America. “Our definition of high socioeconomic status does not guarantee that all women in this analysis have financial stability,” she said, pointing out that the study’s definition of high socioeconomic status was insensitive to wealth accumulation. She also noted that high educational attainment does not necessarily correlate with high income. Hence, the potential burden of economic stressors could not fully be captured.
The study was supported by the National Institutes of Health. Dr. Johnson reported no conflicts of interest.
SOURCE: Johnson J et al. Obstet Gynecol. 2020 Jan;222(1):S-37-8, .