Many sociodemographic risk factors associated with low birth weight have been well documented, such as extremes of childbearing age, smoking, inadequate prenatal care, urban poverty, as well as black race. While the rate of low birth weight declines in both blacks and whites as the number of identified risk factors decrease, the improvement occurs more rapidly among white women than among black women. As a result, the birth weight gap between low risk blacks and whites is wider in comparison to the gap between high-risk blacks and whites. Many have concluded that the birth weight differential is the result of genetic differences between the two races. To explore the genetic connection to birth weight, African-born women residing in U.S. were contrasted to both blacks and whites born in the U.S., since approximately 75% of U.S.-born blacks have West African origin. Vital records from 1989 through 1995 from Illinois were used to evaluate the distribution of birth weight among infants born to three populations; U.S.-born blacks, sub-Saharan African-born blacks, and U.S.-born non-Hispanic Whites.

Only singleton infants were included in the study. The mean birth weight of 44,046 infants of U.S.-born white women was higher than the mean birth weight of 3,135 infants of African-born women, which in turn was higher than the mean birth weight of 43,322 infants of U.S.-born black women. While the rate of low birth weight was 4.3% among infants of U.S.–born white women, the rate was 13.2 % among infants of U.S.-born black women, approximately 3.7 times higher than the white group. However, the incidence of low birth weight was 7.1% among infants of African-born women, which is only 1.65 times higher than the white group and almost half the incidence of low birth weight among U.S.-born black women. In contrast, the incidence of very low birth weight of African-born blacks was comparable to the incidence among the U.S.-born black women. The profile for women at lowest risk of low birth weight infants was characterized by women in their 20s and 30s, who began prenatal care during first trimester, had at least 12 years of education, and were married to men who also attained at least 12 years of education. 66% of the white women, 50% of African-born women, and only 14% of US-born black women matched this low risk profile. Among women with the lowest risk profile, the incidence of low birth weight declined in all three populations. The differences in mean birth weight and rates of low and very low birth weight between infants of African-born women and infants of white women narrowed, while the disparities between infants of whites and U.S.-born blacks were generally unaffected.

The mean birth weight and prevalence of low and very low birth weight among infants of African-born women fell between the those for U.S.-born white and U.S.-born black women. The genetic heritage of U.S.-born black women is approximated to be 75% derived from African origins and 25% from European origins. If genetic significantly contributes to the racial differences in birth weight, then one would expect to observe a narrower gap among the U.S.-born white and black infants than among infants of U.S.-born white and African-born women. However, the authors observed the reverse result. Thus, the findings are inconsistent with the genetic hypothesis. Furthermore, most of the identified risk factors for adverse pregnancy outcomes are limited to factors that are present close to the timing of the pregnancy. Perhaps more appropriately, factors that reflect past life cumulative experience should be considered to obtain a more accurate and clinically practical risk profile for pregnant women, particularly the U.S.-born black women.