This study examined the relationship between urban green space and the weight of newborns in two diverse US cities. Birth weight is one indicator of a newborn's health. Previous studies exploring relationships between green space and birth outcomes have reported inconsistent findings. Additionally, past research exploring this relationship relied primarily on just one type of green space measurement – typically, the satellite Normalized Difference Vegetation Index (NDVI). This study differed by using multiple green space measures: satellite greenness index, percent of green space, percent tree cover, percent tree street buffering, and access to parks, within 50, 250, 500 and 1000 meters surroundings mothers' home.
Additional data for this study included measures of traffic air pollution, population density, and neighborhood socioeconomic factors for each newborn household. This data, along with the green space measurements, were then matched to data of 90,265 newborns in Portland, OR and 88,897 newborns in Austin, TX.
The different green space measurements used for this study produced similar results in terms of urban residents' exposure to green space. Findings also indicated that Portland and Austin offered similar green space exposure. Population and contextual characteristics of the two cities, however, were significantly different. Austin had a greater percentage of mothers who were Hispanic, who had not completed a high school education, and who were below the age of 19. Birth weights were also slightly lower in Austin than in Portland.
For both cities, green space exposure was consistently linked with healthier birth weights. When maternal race/ethnicity and education were considered, however, this association no longer held. Yet, the positive link between green space and birth weight remained significant for the high-density areas of both cities within the smallest buffer of 50 meters surrounding the mothers' homes. The authors suggest that future studies consider the possibility of a population density threshold effect by which greenness matters for higher but not lower density areas.
In contrast to some prior studies, this study did not find evidence for a relatively greater impact of greenness on birth weights for racial/ethic minorities, the less educated or the less advantaged.
Overall, this research suggests that individual and neighborhood characteristics can modify the impact of green space on birth weight, and in different ways depending on the green space metric.The authors discuss the importance of considering multiple measures of greenness, multiple geographic areas, as well as a range of distances from mothers' homes. The authors reported substantially different changes in associations between different measures of greenness and birth weight once models were statistically adjusted for individual and neighborhood characteristics, even though the measures of greenness were highly correlated. This suggests that different green space metrics may represent different pathways of influence on birth outcomes.
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