Racial Disparities at Birth:The Puzzle Persists

A baby born to an African-American (black) mother in the United States is twice as likely to die before reaching her first birthday as a baby born to a European-American (white) mother. A range of conditions contribute to infant mortality, but the most powerful predictors are being born too early (before 37 completed weeks of pregnancy) and/or too small (with a birth weight of less than 2,500 grams). Black infants are two to three times as likely as their white counterparts to be born prematurely and/or with low birth weight. Premature or low–birth weight infants who survive beyond infancy are far more likely than other infants to suffer major developmental problems, including cognitive, behavioral, and physical deficits during childhood, with lasting consequences in adulthood. They also have poorer prospects for employment and wages as adults. Prematurity and low birth weight (together referred to as adverse birth outcomes) also predict poor adult health, including diabetes, high blood pressure, and heart disease, all of which raise risks of disability and premature mortality. Caregiving to chronically ill and/or disabled survivors of adverse birth outcomes is a tremendous economic burden on families and society.

A growing body of research has been conducted in recent years into the causes of the racial disparities. The research has examined a wide range of possible factors, including differences in prenatal care, differences in women’s health before they become pregnant, and infections. This research has produced useful insights but has not identified a clear cause for racial disparities. More recently, researchers have hypothesized a role for stress and adverse experiences throughout life, not just during pregnancy, as possible explanations. Much greater research investment is necessary if we are going to solve the puzzle of why racial disparities in birth outcomes persist.

At least in one major area there is now a strong scientific consensus: Differences in prenatal care are unlikely to explain racial disparities in prematurity and low birth weight. Black/white disparities in receipt of prenatal care have narrowed markedly over time, particularly with major expansions of Medicaid maternity care coverage beginning around 1990, without concomitant narrowing of birth-outcome disparities. In addition, a number of studies have failed to link prenatal care, as typically provided in the United States, to improved birth outcomes in general. The literature is inconclusive regarding effects on birth outcomes of prenatal care enhanced with various forms of psychosocial support; few studies have been conducted that meet rigorous criteria.

Given the scientific consensus that standard prenatal care does not hold much hope for reducing racial disparities in birth outcomes, there has been an increasing interest in focusing on the health of women before they become pregnant, including ensuring access to medical care for chronic conditions. It seems unrealistic to think that medical care given during a nine-month or shorter period could dramatically reverse the adverse effects of a lifetime of experience before conception. It also seems unlikely that medical care alone in the period before conception could reverse the effects of a lifetime of social disadvantage.

Well-established causes of being born too small or too early—without consideration of racial disparities—include prenatal exposure to tobacco, excessive alcohol, or illicit drugs; being underweight at the beginning of pregnancy and gaining insufficient weight during pregnancy; very short maternal stature; and chronic diseases. The known causes of low birth weight and/or preterm birth, however, do not explain the black/white disparities; studies taking these factors into consideration have not seen a narrowing of the racial gap in outcomes. For example, black women are less likely to smoke or to binge drink during pregnancy and less likely to be underweight before pregnancy than are white women.

Several factors have been hypothesized to explain birth-outcome differences by race. Among the more widely held hypotheses has been the notion that occult (hidden) infections may explain the racial gap. Rates of infection with bacterial vaginosis, a genital tract infection previously thought to be benign but recently associated with adverse birth outcomes, are higher among African-American women, as are periodontal infections. Although many clinicians have been optimistic that infections would turn out to be an important and relatively easily modifiable missing piece of the puzzle, treating infections has not consistently led to improved birth outcomes. This suggests that rather than infections being a cause of adverse birth outcomes, they may be a marker for some other factor or factors that are associated with both infections and adverse birth outcomes.

There has been a widespread assumption, without evidence, that genetic differences are the key to the black/white disparity in birth outcomes. In part, this assumption has rested on the observation that the black/white birth-outcome disparities have persisted even after taking into account mothers’ educational attainment or family income around the time of pregnancy. However, no one has identified a gene or genes that are clearly linked to either prematurity or low birth weight, and the mechanisms involved appear different for the two outcomes and complex for both. It is likely that if genetic differences are involved in either, they would involve complex arrays or cascades of multiple genetic factors very unlikely to sort themselves out according to race. Although it is possible that genetic factors, particularly gene/environment interactions, could be involved, a primary role for genes is not supported by observed social patterns, which are discussed below.

Furthermore, current income and education reflect only a small part of the socioeconomic experiences of a woman, which could affect her birth outcomes through a range of biological and behavioral pathways. For example, among U.S. blacks and whites overall, the median net worth of whites ($86,573) is almost 4 times that of blacks ($22,914). In the bottom quintile of income, the median net worth of whites ($24,000) is 400 times that of blacks ($57). Wealth is probably more important than income for health because it can buffer the effects of temporarily low income, providing security as well as a higher standard of living. Furthermore, a black woman of a given educational or income level is far more likely than her similar–education-or-income white counterpart to have experienced lower socioeconomic circumstances when growing up. She also is far more likely to live (and to have lived in the past) in a neighborhood with adverse socioeconomic conditions, such as exposure to environmental toxins, crime, lack of sources of healthy foods and safe places to exercise, and/or poor-quality housing. There are many unmeasured socioeconomic differences between blacks and whites even in studies considering income and education; such studies should not but unfortunately often do conclude that observed racial differences must be genetic since they have “controlled for socioeconomic status.”

Social patterns may give us valuable clues to the unsolved mystery of black/white disparities in birth outcomes. For example, although birth outcomes consistently improve with higher education or income, the relative disparities are largest among more affluent, better-educated women: nearly a threefold difference in our data from California and also observed to be large in national data. The racial disparity is also seen among poor and uneducated women, but it is much smaller, closer to 1.3 in 1 in recent study. Why would the racial disparity be greater among higher–socioeconomic status (SES) women? It is unlikely that higher-SES black women are genetically more different from their white counterparts than are lower-SES women. (This issue is discussed further below.)

Comparisons among black women according to birthplace may also provide important clues to likely and unlikely causes of the disparities in birth outcomes. Mirroring what has been called the “Hispanic paradox” of good birth outcomes for immigrant Hispanic women (despite poverty) and poor birth outcomes of their U.S.-born daughters (whose income and education levels are generally higher around the time of childbirth than those of their immigrant mothers), black immigrants also have better birth outcomes than U.S.-born black women. In contrast to the unfavorable (compared to whites) birth outcomes of black women born and raised in the United States, birth outcomes among black immigrants from Africa and the Caribbean are relatively favorable, especially after considering their income and education. As with the comparison of racial disparities in different socioeconomic groups noted above, it is very difficult to explain this disparity by maternal birthplace with genetic differences. If the basis for the differences in birth outcomes by maternal birthplace were genetic, one would expect the immigrants (presumably with a heavier “dose” of the adverse genes) to have worse outcomes, not better.

Stress: A key piece of the puzzle?

In the past 15 to 20 years, knowledge has accumulated about the physiologic effects of stress, particularly chronic stress, in explaining racial differences in birth outcomes. Chronic stress could lead to adverse birth outcomes through neuroendocrine pathways. Neuroendocrine and sympathetic nervous system changes caused by stress could result in vascular and/or immune and inflammatory effects that could lead to premature delivery as well as inadequate fetal nutrition. Living in a crime-ridden neighborhood and facing constant pressures to cope with inadequate resources for housing, child care, transportation, and feeding and raising one’s family are stressful, but such factors are rarely measured. Racial disparities in wealth and income are likely to translate into racial disparities in social networks that can provide financial and other material support during times of need. A growing body of literature on the health effects of subjective social status suggests that an awareness that one is in a group considered socially inferior could be a stressor with strong health effects.

Studies of stress as a possible contributor to adverse birth outcomes have not produced consistent findings. They have, however, tended to focus on stress experienced during pregnancy, rather than chronic stress across a woman’s lifetime, despite the fact that current knowledge of the health effects of stress makes chronic (rather than acute) stress far more plausible as a causal factor in racial disparities in health. It could be a key mediator of many of the unmeasured socioeconomic factors that vary by race, including childhood socioeconomic adversity and neighborhood socioeconomic conditions.

It is biologically plausible that experiences associated with a legacy of racial discrimination are another potential source of unmeasured stress that may contribute to black/white disparities in birth outcomes, and some studies have demonstrated this connection. Incidents of overt racism against African-Americans in the United States are still pervasive, although probably becoming less frequent over time. More subtle experiences associated with racism, however, also could be stressful; for example, a constant awareness and state of arousal in anticipation of racist comments, whether subtle or overt, being made in one’s workplace could be stressful. Vicarious experiences related to fears about one’s children or other family members facing discrimination; or a background awareness of the long history of discrimination, including slavery, experienced by blacks in general; are other potential sources of chronic stress that also could exact a health toll, including on birth outcomes, even in the absence of overt incidents. The literature in this area is in the very early stages of development, and the results are not consistent; better measures of experiences of racism are needed to advance knowledge of the potential health effects of discrimination in various forms, not only dramatic overt incidents.

Could experiences of racism account for the counterintuitive finding of a greater racial disparity in birth outcomes among more affluent and educated women? One can only speculate, but unmeasured differences in socioeconomic factors during life appear to be a possibility, along with experiences related to racial discrimination. Unmeasured socioeconomic exposures (for example, in childhood and/or at the neighborhood level) could influence birth outcomes through pathways involving nutritional effects, exposure to toxins, and other adverse exposures related to low socioeconomic status, as well as stress. Paradoxically, a more educated black woman may, on a chronic basis, experience more discrimination and more constant awareness and fears of it, because she is far more likely than her less educated black counterpart to be working, playing, shopping, and traveling in a predominantly white world.

Implications for action

Given the staggering influence of birth outcomes on health during lifetimes, far more investment is needed in understanding the mechanisms that explain prematurity and low birth weight and racial disparities in them. Far more research is needed on social and psychological influences on birth outcomes, on how they are mediated biologically, and on how to intervene even before we completely understand all of the mechanisms at the molecular level. We have no firm answers now (except perhaps firm indications about some disproven explanations), but we have some very plausible hypotheses that require testing under a range of circumstances. Among the biologically plausible hypotheses are a major role for stress and adversity experienced throughout life, not only during pregnancy, which would mean that intervening during pregnancy may be too little and too late. Unmeasured experiences in early childhood and across a woman’s life before conception could be important sources of stress that could explain racial disparities. These experiences could include unmeasured socioeconomic factors at the neighborhood and family levels as well as experiences related to racial discrimination and awareness of it, even in the absence of dramatic overt incidents. Gene/environment interactions cannot be ruled out as contributors to racial disparities in birth outcomes. If these interactions were involved, however, they would be very complex; biomedical solutions are not on the horizon at present and in any case would be a long way off, making it important to make vigorous efforts to identify and modify the triggers for the disparity in the social and physical environments. It makes scientific sense to focus on social advantage and disadvantage—including not only socioeconomic factors but also potentially subtle, chronically stressful experiences related to our legacy of racial discrimination—as plausible contributors to black/white disparities in birth outcomes. Even without definitive proof of their role in birth-outcome disparities, there are compelling ethical and human rights reasons to direct our attention to eliminating the profound and longstanding differences in social conditions that still break down along lines of skin color.