Special Article

Differing Birth Weight amidst Infants of U.S.-Born Blacks, African-Born Blacks, and U.Southward.-Born Whites

List of authors.
  • Richard J. David, Chiliad.D.,
  • and James Westward. Collins, Jr., Thou.D., M.P.H.

Abstract

Background

In the United states, the birth weights of infants of black women are lower than those of infants of white women. The extent to which the lower birth weights among blacks are related to social or genetic factors is unclear.

Methods

We used vital records for 1980 through 1995 from Illinois to determine the distribution of birth weights amidst infants born to three groups of women — U.S.-born blacks, African-born blacks, and U.Southward.-born whites.

Results

The mean birth weight of 44,046 infants of U.Southward.-born white women was 3446 g, that of 3135 infants of African-born black women was 3333 1000, and that of 43,322 infants of U.S.-born black women was 3089 m. The incidence of low birth weight (weight less than 2500 m) was 13.ii per centum among infants of U.S.-born blackness women and vii.1 percent among infants of African-born black women, as compared with four.iii percentage among infants of U.S.-built-in white women (relative risks, 3.i and 1.6, respectively). Amidst the women at lowest risk (those 20 to 39 years old, with 12 years of education for themselves and their spouses, early on prenatal care, gravida two or 3, and no previous fetal loss), the charge per unit of low nascency weight in infants of African-built-in black women (3.6 percent) was closer to the rate in infants of U.South.-born white women (two.4 percent), and the rate in infants of U.S.-built-in blackness women remained loftier (vii.5 per centum).

Conclusions

The birth-weight patterns of infants of African-born black women and U.Due south.-built-in white women are more closely related to one another than to the birth weights of infants of U.S.-born black women.

Introduction

During the by forty years, epidemiologic inquiry has elucidated many of import associations betwixt the sociodemographic characteristics of mothers and the nascency weight of infants.1-4 For case, the extremes of childbearing age,1 cigarette smoking,2 inadequate prenatal intendance,iii urban poverty,iv and black race5 are well-documented risk factors for low nativity weight. Other obstetrical risk factors account for part of the racial disparity in nascency weights, simply differences persist.6-ix

Although the incidence of depression birth weight decreases in both blacks and whites as the number of risk factors declines, the comeback is faster among whites, resulting in a wider birth-weight gap between blacks and whites amongst infants of low-risk women.1,iv This has led some investigators to believe that genetic factors associated with race influence nascence weight.10-fifteen In the 1967 National Collaborative Perinatal Project, just 1 percent of the total variance in birth weight amid 18,000 infants was accounted for by socioeconomic variables, leading the authors to conclude that "race behaves every bit a real biological variable in its effect on nascency weight. This event of race [is] presumably genetic."ten The assumption that black women differ genetically from white women in their power to bear normal or large infants persists in more recent studies of fetal growth,thirteen,xvi one of which, for example, refers to "genetic factors affecting growth, such as neonatal sex activity and race."sixteen

Few information have been published on the birth weights of infants born to African-built-in women in the United States. Nigh African Americans trace their origins to western Africa, where the slave merchandise flourished in the 17th and 18th centuries.17,18 It is estimated that U.S. blacks derive about three quarters of their genetic heritage from West African ancestors and the remainder from Europeans.eighteen-21 To the extent that population differences in allele frequency underlie the observed differences in birth weight between blacks and whites in the United states, 1 would expect women of "pure" West African origin to behave smaller infants than comparable African Americans, considering the European genetic admixture in the latter. Still, to our knowledge, no population of Westward African women delivering infants in the United states of america has been studied. We therefore undertook an analysis of racial differences in nativity weight based on U.S.-born and African-born women giving birth in Illinois.

Methods

Report Population

Nosotros obtained data on the nascency weights of singleton black and white infants built-in in Illinois and the birthplaces of their mothers, using birth-document tapes for 1980 through 1995 from the Illinois Department of Public Health. All the white infants studied had U.South.-born mothers and were not of Latino origin. The mothers of the black infants fell into ii groups: women born in sub-Saharan Africa and those built-in in the United States. We selected random samples of the white and black U.S.-born women in social club to have groups convenient for analysis; these groups included two.5 percent of white births and vii.5 percent of black births.

Blackness women built-in in the Western Hemisphere but not in the U.s. (i.due east., built-in in Canada, the Caribbean area, or Southward America) were excluded from the report. Such designations of maternal origin were available for the flow 1980 through 1988. During that period, nascence records were coded with three split up fields: the mother's race, the mother'due south identify of birth, and the female parent's origin or descent. Women whose race was coded as "black," whose place of birth was coded as "not in Western Hemisphere," and whose origin or descent was coded equally "Africa, excluding northern Africa" were considered to have immigrated from sub-Saharan Africa. Co-ordinate to the 1990 Census, 66 percent of African-born blacks living in Illinois for whom a sub-Saharan country of birth was recorded came from either Nigeria or Ghana.22 From 1989 on, the variable indicating origin or descent was replaced by a variable specifically pertaining to Hispanic origin, but a new, detailed set up of birthplace codes allowed united states to identify births on the basis of the female parent'due south land of birth. We therefore selected births from 1989 through 1995 in which the mother's birthplace was one of 17 nowadays-day countries corresponding to the area from which African slaves originated in the 17th and 18th centuries.18,20

Analysis of Birth Weights

Every bit a first stride toward exploring the possible contribution of genetic factors to the racial disparity in outcomes of pregnancy, nosotros compared the curves for the distribution of birth weight, the mean birth weights, and the rates of low birth weight (defined as the number of births of infants weighing less than 2500 thousand per 100 live births) of infants born to U.Due south.-born blacks, African-built-in blacks, and U.S.-born whites. In improver, nosotros computed rates of moderately low (1500 to 2500 chiliad) and very low (<1500 m) birth weight. Next, nosotros determined the distribution of sociodemographic take a chance factors (the mother's age, education, and marital status, the trimester of first prenatal care, and the father's education) and reproductive take a chance factors (the overall number of pregnancies and whether there was a history of fetal loss or infant death) in the 3 groups of women. For the take a chance factors and outcomes, we calculated relative risks and 95 pct confidence intervals, using the infants of U.S.-built-in white women every bit the reference group.23

Because the 3 populations differed, we repeated the nascency-weight comparisons after aligning for differences in hazard profiles. We did so in iii ways. Get-go, we compared each African-built-in mother with 2 similar U.South.-born women, ane white and one black, who were matched for age, education, marital status, prenatal intendance, parity, and history of fetal loss. Second, nosotros used the REG procedure (SAS, release 6.07, Cary, N.C.) to create a model showing birth weight every bit a function of all the take a chance factors for which data were available, except paternal education (data on that variable were missing for 20 percent of births) and prior loss of an babe (prevalence, <5 pct). We then estimated mean differences in nativity weight among the three subgroups, both past subtracting intercept terms estimated in three subgroup-specific models and past modeling the subgroups two at a time, with ethnic condition entered as a dichotomous dummy variable.24 Third, we repeated the birth-weight analysis just limited it to subgroups of depression-risk women defined according to social, demographic, and reproductive risk factors.

Our assay used birth-certificate tapes from which the identifying information on the individual women and their infants had been removed. These data were provided by the Illinois Department of Health, which provides such "sterilized" birth tapes to researchers conducting epidemiologic studies.

Results

Table 1. Table 1. Nativity-Weight Data in Illinois, 1980–1995, According to the Mother's Race and Identify of Birth. Figure ane. Effigy 1. Distribution of Nativity Weights among Infants of U.S.-Born White and Black Women and African-Born Black Women in Illinois, 1980–1995.

The calculation of frequencies was based on all singleton births in Illinois. The study population included the infants of 3135 black women born in sub-Saharan Africa, 43,322 black women built-in in the Usa (a sample that included seven.5 percentage of the full number of blackness women giving birth in Illinois), and 44,046 U.S.-born white women (2.five percentage of the total number of white women giving birth in Illinois).

The mean birth weight of the white infants was 3446 k, as compared with 3333 grand for the infants of the African-built-in black women and 3089 1000 for the infants of the U.South.-born blackness women (Tabular array i). The proportion of very-low-birth-weight infants was similar for African-built-in blacks and U.S.-born blacks. Even though the infants born to African-born blacks had a slightly lower hateful birth weight than the white infants, the overall distribution of birth weights was like in the two groups and was different from that among the infants of U.Due south.-born blacks (Effigy 1).

Table 2. Table 2. Distribution of Selected Take a chance Factors in the Study Population According to the Mother's Race and Place of Nascence.

Table 2 shows the distribution of selected risk factors in the 3 groups of women. The African-born black women delivered the highest proportion of infants who were their mothers' quaternary or subsequent children and had the highest proportion of previous fetal and infant deaths. The U.S.-born blackness women were the youngest, the least probable to be married, the least well educated, and the most likely to have received prenatal care belatedly or non at all. The white women surpassed both groups of black women with regard to only one run a risk gene — primigravidity.

When the infants of African-built-in black women were compared with those of U.S.-born women matched for the mother'south age, marital status, education, prenatal care, parity, and prior fetal loss and the male parent's instruction, the differences between the groups narrowed somewhat, simply their relation did not change (Table ane). With white infants as the reference grouping, the relative risks for low and moderately low birth weight were both significantly higher among infants of U.S.-born blacks than among infants of African-born blacks. However, the relative adventure of very low birth weight was similar in the ii groups of infants born to blacks.

Table 3. Table 3. Regression Models Showing the Predicted Effects of Low-Hazard Sociodemographic and Reproductive Variables in the Mother on the Nascence Weight of Infants in Each Subgroup Defined According to the Mother's Race and Place of Birth.

To gain more insight into the relative importance of the gamble factors in the three groups, we used multiple-regression assay to written report the changes in birth weight predicted by each factor. The models we synthetic (Table 3) all showed a positive effect of being married (an increment of 60 to 124 g in predicted birth weight), having had one or two previous pregnancies (an increase of 29 to 50 thousand), and having no previous fetal loss (an increment of 19 to 55 thou). Of the chance factors, only marital status had a statistically significant effect amongst the infants of African-born blacks.

On the footing of the multivariable models in Table 3, the nativity weight of the infants of African-born blacks was 14 g less than that of the infants of U.S.-born whites after we controlled for take a chance factors. In some other model, we looked at simply the U.S.-built-in white women and the African-built-in black women, with race included as a dichotomous variable. In that analysis, the infants of the U.South.-born whites weighed 98 g more the infants of the African-born blacks after adjustment for historic period, education, marital status, gravidity, prenatal care, and a history of fetal loss. In a similar model that included only women built-in in the United states, the white infants weighed 248 g more than the blackness infants after aligning for the same half-dozen variables.

Table 4. Table iv. Mean Birth Weights and Rates of Low Birth Weight amid Infants with Mothers at Low Hazard, Co-ordinate to the Mother's Race and Place of Birth.

Table 4 shows the mean birth weights and rates of low nativity weight among infants born to the women at everyman take a chance — those twenty to 39 years of historic period who began their prenatal care in the first trimester, had at least 12 years of instruction, and were married to men who likewise had at least 12 years of pedagogy. Sixty-six pct of the white women fit this profile, as compared with 50 percent of the African-born black women and xiv per centum of the U.South.-born black women. The mean nativity weight and rates of low birth weight of the infants born to African-born blacks were intermediate between the values in U.S.-born whites and those in U.Due south.-born blacks. Nevertheless, when reproductive risk factors were included in the selection of low-hazard women, the differences between the infants of U.S.-born whites and the infants of African-built-in blacks in hateful nascency weight and rates of both low and very depression nativity weight were narrowed, whereas the differences between the infants of U.S.-born whites and U.Southward.-built-in blacks were unchanged. The greatest change was in very low birth weight; the exclusion of women with a history of fetal loss resulted in nearly identical rates amidst infants of African-born blacks and those of U.S.-born whites, eliminating the pregnant backlog of infants with very depression birth weight born to African-born blacks.

Discussion

The distribution of nativity weights among infants of African-built-in black women approximated that among infants of U.S.-born white women. The charge per unit of depression-nascence-weight births for African-built-in black women was between the charge per unit for U.S.-born white women and that for U.Southward.-built-in black women. Adjusting for maternal risk factors in three ways shifted the magnitude of the differences in birth weight simply did not alter the basic pattern. Amongst infants of African-born black women and those of U.Due south.-born black women, very low birth weight occurred at a similar frequency. Nevertheless, these information provide some evidence against the theory that at that place is a genetic ground for the disparity betwixt white and black women born in the U.s.a. in the hateful nascence weights of their infants.

Co-ordinate to most studies, racial differences in birth weight persist independently of numerous social and economic risk factors.eight,ix This has led some investigators to suggest that the differences have a genetic ground.eleven-14 Our findings challenge the genetic concept of race every bit it relates to birth weight. The African-born women in our study were new immigrants from the same region from which the ancestors of near U.Due south. blacks came, but without the estimated xx to 30 percent admixture of European genetic material that has occurred since the mid-17th century.18-21 If genetics played a prominent role in determining black–white differences in birth weight, the infants of the African-built-in black women should accept had lower nativity weights than those of the U.South.-born black women. We institute the opposite: regardless of socioeconomic condition, the infants of blackness women born in Africa weighed more the infants of comparable black women built-in in the United States.

The birth-weight distribution of the infants of African-born black women who delivered in Illinois is consistent with previous reports of the birth weights of infants of foreign-born blackness women of largely Caribbean origin.25-28 Studies of groups of women from New York, Boston, and multiple states have had concordant results: black women built-in outside the United States have heavier infants than those born inside the United states of america, even after adjustment for cigarette smoking, alcohol intake, and illicit-drug use.

Every bit information inconsistent with the genetic hypothesis of racial differences accumulate, social and psychophysiologic hypotheses are advanced.5,29-33 A woman's exposure as a young child to the furnishings of poverty or racial discrimination could adversely touch birth weight in the next generation.28,34 The high educational level of African-born blackness women in Illinois indicates that rigorous selection occurs amongst African immigrants and suggests an overrepresentation of women born into affluent families, an elite subgroup in any developing nation.

Wilcox and Russell, in their extensive piece of work on nascency-weight distributions, developed a model that can exist applied to the nascency-weight curve of whatsoever group, partitioning information technology into an underlying gaussian curve and a "residual" distribution of very-low-birth-weight infants.35 They proposed that the definition of normal birth weight differs for different groups, on the footing of the underlying distribution in the grouping under consideration. They attribute the residue births of very-low-nativity-weight infants to "disorganized, perhaps pathologic, processes"35,36 that are presumably ecology in origin.

In our study, the proportions of very-depression-birth-weight infants born to African-born black women and to U.S.-born black women were similar. The factors that account for this finding are unclear. As in most published studies, the majority of the risk factors nosotros examined were related to the form of pregnancy. In such a conceptualization, pregnancy is a relatively short-term condition, minimally related to past life experiences. In an endeavour to augment this concept, we studied how the consequence of prior pregnancy affected the disparity between blacks and whites in rates of very low birth weight. When we controlled for the outcome of prior pregnancy, we found that the rate of very low nascency weight among infants of African-built-in blackness women more closely resembled that amongst infants of U.Southward.-born white women. This observation deserves further investigation.

Our study has important limitations. Vital records contain minimal clinical information. Data on cigarette smoking, weight before pregnancy, and weight gain during pregnancy might, if available, have explained some of our findings. In improver, the grouping of African-born black women studied, although more x times larger than the group studied previously,37 was too pocket-size to permit stable estimates of very depression nascence weight in subgroups.

In summary, African-born blackness women accept infants with a greater hateful nativity weight and a different birth-weight distribution than blackness women born in the Usa.

Funding and Disclosures

We are indebted to Mr. Steven Perry and the staff of the Illinois Department of Wellness for providing vital-records data; to Mr. James Bash and Ms. Barbara Sullivan for technical assistance; to Drs. Ugonna Chike-Obi, Richard Cooper, Helen Kusi, and Adeyemi Sobowali for useful comments; and to Ms. Susan Seidler for help in the preparation of the manuscript.

Writer Affiliations

From the Segmentation of Neonatology, Cook County Children's Hospital (R.J.D.); the Section of Pediatrics, School of Medicine, Academy of Illinois at Chicago (R.J.D.); the Sectionalization of Neonatology, Children's Memorial Infirmary (J.W.C.); and the Section of Pediatrics, Northwestern University Medical School (J.W.C.) — all in Chicago.

Accost reprint requests to Dr. David at the Division of Neonatology, Cook County Children's Hospital, 700 Southward. Forest St., Chicago, IL 60612.

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