Neighborhood-level factors appear to have independent effects on the birth weight of babies born to urban residents. According to a study based on individual- and neighborhood-level data from 343 Chicago neighborhoods,1for white mothers only, the more socially cohesive and supportive a woman's neighborhood, the heavier her baby was. For black mothers only, increasing levels of neighborhood economic disadvantage were associated with significant reductions in their babies' mean birth weight.
The data covering 343 Chicago neighborhoods came from three sources. The authors' household survey of 8,782 adults, conducted in 1995 as part of the Project on Human Development in Chicago Neighborhoods, yielded data on levels of perceived neighborhood support; these were based on responses to 10 items assessing social cohesion, trust and interaction (with higher scores indicating higher levels of neighborhood support). Birth certificates for 95,711 singleton infants born in 1994-1996 (to 65,923 blacks and 29,788 whites) provided information on birth weight and maternal risk factors. Finally, data from the 1990 census provided information on economic disadvantage (a composite index of the proportion in poverty, on public assistance and unemployed, with higher scores indicating greater disadvantage).
Five percent of babies born to white women had low birth weight (less than 2,500 g), compared with 13% of babies born to black women. Babies born to whites weighed 3,389 g at birth, on average, whereas those born to blacks weighed 297 g less. A greater proportion of black mothers than of white mothers were unmarried (84% vs. 20%), were teenage mothers (27% vs. 6%) and had had inadequate prenatal care (13% vs. 6%). Lower proportions of blacks than of whites had completed high school (62% vs. 86%) and were giving birth for the first time (26% vs. 39%).
The average birth weight of babies of either race was highest in neighborhoods that were predominantly white (i.e., where more than 90% of births were to white women), and lowest in those neighborhoods that were predominantly black (i.e., where more than 90% of births were to blacks). Likewise, economic and social support measures differed sharply by neighborhood racial composition: Compared with predominantly white neighborhoods, those that were mostly black had a higher proportion of residents living in poverty (31% vs. 6%), a higher mean score on the scale measuring economic disadvantage (0.9 vs. -0.9) and a lower mean score on the scale measuring perceived neighborhood support and cohesiveness (-0.3 vs. 0.6).
The researchers ran several hierarchical linear regression models, estimated for blacks and whites separately, that predicted associations between neighborhood- and individual-level variables and mean birth weight. One controlled for maternal risk factors and infant gender only, the next added neighborhood racial composition and economic disadvantage, and another added neighborhood social support. The results indicate that much of the racial differential in birth weight was explained by individual-level variables: In the model controlling only for maternal risk factors and infant gender, the difference was reduced from 297 g to 154 g; adding controls for neighborhood racial composition and economic disadvantage lowered that difference to 121 g. Further controlling for neighborhood support yielded a birth-weight differential of 124 g.
The effect of maternal risk factors on infant birth weight was generally comparable among babies born to blacks and to whites. For example, in the fully adjusted model, the receipt of adequate prenatal care was independently associated with increases in mean birth weight among both blacks and whites (181 g and 132 g, respectively). Similarly, higher maternal education was related to birth-weight increases among blacks and among whites (20 g and 12 g, respectively); marriage was also associated with increases in birth weight (97 g and 87 g). There was no difference by race in the effect of the infant's gender on birth weight; male newborns of either race weighed 108-112 g more than female newborns. Black women giving birth for the first time experienced smaller decreases in their babies' birth weight than similar white women (reductions of 38 g vs. 79 g). While maternal age had no independent effect on birth weight among babies born to whites, each year older a black mother was when she gave birth, her infant's birth weight decreased by approximately 8 g.
Perceived level of neighborhood support was independently associated with birth weight among whites, but not among blacks. An increase of one standard deviation in the neighborhood support scale was related to a significant 18 g increase in the mean birth weight of babies born to white women. Net of all variables, economic disadvantage was associated with a decrease in birth weight for blacks (15 g for each standard deviation increase in the disadvantage index), but not for whites. The neighborhood racial composition had no effect, however, on mean birth weight among babies born to women of either race.
The investigators acknowledge that their study is limited because it was not possible to pinpoint specific support components potentially associated with improved outcomes, detailed individual-level socioeconomic data were lacking, and an insufficient number of black women lived in supportive neighborhoods to assess whether blacks might accrue the same benefits from such support as whites. The researchers offer several possible mechanisms through which neighborhood support may improve birth weight. A supportive local environment may discourage harmful activities such as prenatal smoking and drug use, provide positive norms of health care-seeking behaviors and directly affect fetal growth by lowering the likelihood of biological response to stress. They conclude that the "growing evidence of the health benefits of neighborhood cohesion, support and engagement" may have important implications for public health policy and practice.--L Remez
1. Buka SL et al., Neighborhood support and the birth weight of urban infants, American Journal of Epidemiology, 2003, 157(1):1-8.