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Journal Issue: School Readiness: Closing Racial and Ethnic Gaps Volume 15 Number 1 Spring 2005

Low Birth Weight and School Readiness
Nancy E. Reichman

Disparities in Low Birth Weight by Race, Ethnicity, and Nativity

The black-white disparity in low birth weight in the United States is glaring and persistent. In 2000, 13 percent of babies born to black mothers were low birth weight, compared to 6.5 percent of those born to white mothers.13 (By contrast, rates of low birth weight for the other racial groups reported by the National Center for Health Statistics were close to that of whites: 6.8 percent among American Indians and 7.3 percent among Asians and Pacific Islanders.)14 The two-to-one disparity between blacks and whites has persisted for more than forty years, exists at most maternal age ranges, cannot be explained by differences in rates of multiple births, and cannot be explained by socioeconomic status alone.15 Even infants born to college-educated black women are at much greater risk than infants born to college-educated white women of being low birth weight.16 Black mothers were 63 percent more likely to have preterm deliveries than white mothers (17.3 percent as against 10.6 percent) in 2000.17 The rates of small-forgestational-age births among infants born at term in 1998 were 17.4 percent among blacks and 9.0 percent among whites.18

Ethnicity
Rates of low birth weight also vary among women of different ancestral origins. The rate for women of Hispanic descent was 6.4 percent in 2000, on par with the rate for whites. But within that broad group, rates differ widely. In 2000, women of Cuban and Mexican descent had low birth weight rates of 6.5 percent and 6.0 percent, respectively, while Puerto Ricans had a rate of 9.3 percent.19 The disparity between Puerto Ricans and Mexicans has baffled researchers because both groups are at high risk for adverse outcomes based on their socioeconomic status, and island-born Puerto Ricans, as U.S. citizens, have greater access than foreign born Mexicans to Medicaid.20 The disparity may have to do with unmeasured differences in culture, diet, stress, or lifestyle.21 Researchers have termed the unexpectedly favorable rates among Mexican American women, despite their socioeconomic disadvantages and comparatively low use of prenatal care, the epidemiologic or Hispanic paradox.22  Explaining this paradox could provide clues about how to blunt the negative effects of poverty on birth outcomes of other disadvantaged groups. Blacks of Puerto Rican or other Hispanic ethnicity have a lower probability of low birth weight than blacks who are non-Hispanic, but very few (3 percent) of the 622,598 births to black mothers in 2000 were to mothers who identified themselves as Hispanic.23

Several researchers, notably Gosta Rooth, have questioned the standard 2,500 gram cutoff for low birth weight, arguing that it does not account for variation in mean birth weights across countries that may be due to differences in, for example, maternal height.24 That threshold may likewise not be appropriate for all racial and ethnic groups in the United States, but the “natural” underlying distributions are not known and may themselves be determined by factors such as health and socioeconomic status rather than biological predisposition. Nigel Paneth, in an excellent summary of this issue, suggests that there is not enough evidence to dismiss the glaring racial disparities in low birth weight in the United States as “normal.”25

Nativity
In 2000 some 80 percent of U.S. births to white women and 88 percent of births to black women were to mothers born in the United States.26 Many groups of immigrant mothers, particularly Mexicans, make less use of prenatal care and other health services than their U.S.-born ethnic counterparts because of multiple legal, language, socioeconomic, and cultural barriers.27 Yet the birth outcomes of Mexican immigrants are even more favorable than those of U.S.-born Mexican mothers. In fact, for virtually every racial and ethnic group in the United States, immigrants have better birth outcomes than U.S.- born mothers.28 Thus, although immigrants encounter numerous barriers to prenatal care, they have offsetting health, social, or lifestyle advantages that promote favorable birth outcomes.

Several studies have analyzed birth outcomes of black women by nativity.29 Of particular interest, Richard David and James Collins found that African-born black mothers have rates of low birth weight much closer to those of U.S.-born white mothers than to those of U.S.-born black mothers of predominantly African descent. This suggests that black-white disparities in low birth weight may be due to social and environmental factors rather than biological predisposition, although one cannot rule out the possibility that the differences are due to selective migration.

Low Birth Weight, Survival, and Race
Given the large disparity in low birth weight between blacks and whites and the small disparities between whites and other racial groups and between whites and Hispanics, in the remainder of this article I focus on black-white differences in school readiness. Whenever possible I focus on the lowest birth weight infants, because although they compose small proportions of all births, they suffer the highest rates of disability and therefore have the poorest long-term prognosis for school readiness and academic achievement. As figure 3 shows, the rate of low birth weight among blacks (single births) was the same in 2000 as in 1980; that for whites increased slightly.30 The black-white disparity occurs across all low birth weight groups but is even larger for the lowest weight groups. And while the overall rates of low birth weight have remained constant, the shares of births in lowest weight groups have increased, particularly for blacks. Between 1980 and 2000 the rate of extremely low birth weight rose almost 50 percent among blacks and a third among whites, while the rate of very low birth weight rose about 25 percent among blacks and 15 percent among whites. These higher rates may reflect increased obstetric intervention that prevents fetal loss. Overall reported fetal deaths at 20 or more weeks' gestation declined 12 percent over 1990–2000 alone; the decreases for non-Hispanic whites and non-Hispanic blacks were 10 percent and 5 percent, respectively.31

The rate of infant mortality (death in the first year) has fallen steadily for both blacks and whites over the past twenty-five years. In 1980, 18 out of 1,000 black singleton babies did not live to their first birthday; by 2000 that figure had fallen to 12 out of 1,000. For white babies the comparable rates were 9 out of 1,000 in 1980 and 5 out of 1,000 in 2000.32 As with low birth weight, the two-to-one black-white disparity in infant mortality has persisted over time, although the percentage decline in infant mortality has been greater among whites than among blacks.

Birthweight-specific survival rates are remarkably equivalent for black and white singletons. In the past, black low birth weight infants had a paradoxical survival advantage, perhaps owing to differences in fetal health and differential rates of fetal loss. In 1980, 83 percent of black and 76 percent of white singleton infants of very low birth weight (here, 1,000–1,499 grams) survived their first year; for extremely low birth weight infants, the survival rates were 29 percent for blacks and 27 percent for whites. In 2000, survival rates for very low birth weight infants were 93 percent for whites and 94 percent for blacks; and for extremely low birth weight babies, 58 percent for whites and 57 percent for blacks. Even taking into account multiple births, recent figures show no indication of racial disparities in birth weight–specific survival or in birth weight–specific neonatal survival (the first 28 days of life).33 The lifesaving advantages of neonatal care thus appear to be color-blind, at least in the aggregate. (These figures do not speak to whether there are disparities in newborn care more generally.) However, because black infants are much more likely to fall into the lowest weight groups, a disproportionate fraction of black survivors is at high risk for adverse health and developmental outcomes.

Among survivors born in 2000 (including multiple births), the share of black infants who were extremely low birth weight is 1.00 percent, more than three times that for whites (0.32 percent). The difference is similar for very low birth weight babies (2.31 percent for blacks, as against 0.89 percent for whites).34 Thus among children born in 2000 who survived their first year of life, black children are more than two and a half times as likely as white children to have been extremely or very low birth weight—and therefore to be at risk of serious cognitive delays that could affect school readiness and academic achievement when they enter kindergarten in 2005.35