Understanding links between social vulnerability and adverse birth outcomes for each year (2000-2015) required 15 MLR models for each outcome measure (LBW and PTD). While trends in variable interactions across all years would clearly indicate key drivers, this analysis primary aim is a more holistic understanding of all interactions. Annual MLR model runs controlling for all other social vulnerability variables enables identification of individual variable interactions year-to-year. Although some threads of similar socioeconomic influence are seen across each annual model run, there are many instances where adverse birth outcome drivers vary year to year. Furthermore, social variables are grouped according to their theoretical link to vulnerability, known here as vulnerability "Pillars." These pillars categorize the indicators into concepts, each pillar showing the underlying dimensions of the SoVI index [32].
Across all models, the pseudo-R-square values range from .104 to .304, indicating low to moderate overall model fit across the years and outcomes. The data has a slightly higher fit for LBW in 2009 (Nagelkerke Psuedo R2 of .304) than other years; however, generally, lower pseudo-R-squared values suggest that there are many additional variables besides social vulnerability driving adverse birth outcomes. However, because the intent of this analysis is to build an understanding of social vulnerability characteristic influence on adverse birth outcomes rather than developing a complete model for predicting birth outcomes, such Nagelkerke Psuedo R2 value are expected. In this way, individual variable odds ratios and associated significance produced by MLR suggest that several social variables each year have a substantial influence on adverse birth outcomes. Tables 5 and 6 show MLS model information, including number of inputs, Chi-Square significance, Nagelkerke Psuedo R2 for each year/model, and those social vulnerability variables with a significant influence on adverse birth outcomes.
Low Birth Weight Models
Many social vulnerability indicators provide a significant and robust influence on low-birth-weight rates across the study area (Table 5). Twenty-six different social vulnerability indicators were influential in predicting low birth weight Rates across the SE United States from 2005-2015. While some of these social indicators were only significant in a limited number of model runs, several characteristic groupings (low, medium, high percentages) were predictive in most models (ie. Low Hispanic Populations was a significant and robust indicator in 75% of models, Mobile homes (50% of models), educational attainment (56% of models), female-headed households (50% of models), and renters (50% of models) (Table 5A).
Racial and ethnic variables were among the most frequent influential social vulnerability indicators of low birth weights in the Southeast United States between 2000-2015. Counties have an increased likelihood (+42% - +66% likelihood) of higher low-birth-weight rates when they also have low and medium percentages of Hispanic populations and (+25 - +77%) when a county had at least medium percentages of Native American populations compared to higher percentages. Similarly, between 2000 – 2005, counties with low and medium-low percentages are age-dependent populations (under 5 or over 65 years) had increased likelihood (+44% - +66%) of higher LBW rates than counties with higher percentages of age-dependent populations. These results indicate a protective effect associated with higher populations of these racial and ethnic populations. Further, although a suite of socioeconomic indicators shows the influence on LBW rates in some years, per-capita income (a routinely used indicator) was a less robust indicator of LBW rates across the study area in comparison to housing value. Here, house value provides the most consistent wealth indicator of LBW across many years. Like race and ethnicity, counties with low and medium house values have a higher likelihood of low LBW compared to counties with higher house values.
Conversely, several social vulnerability indicators show a substantial and significant positive influence on LBW. Namely, counties with low and medium percent Black populations, females, female-headed households, educational attainment, unemployment, extractive and service employment, renters, limited English proficiency, and social security beneficiaries tended to have lower LBW rates in comparison to counties with high percentages of these characteristics (Table 5B). Unfortunately, some of these findings point to inequities requiring immediate attention and solutions. Each of these "positive influences" points out that counties with the highest percentages across these social vulnerability indicators are more likely to have higher LBW rates. This sad fact requires swift intervention.
Preterm Delivery Models
Twenty-four (24) social vulnerability variables were influential in one or more PTD models for the SE United States (Table 6). Like LWB models, several variables were only significantly influential in one or few models, included the Percentage of People Living in Poverty, which was only a significant predictor in the 2000 and 2004 models. However, several groupings of variables, including low/medium percentage black populations (81% of models), low/medium gross rent (43% of models), and low/medium nursing home residents per capita (37% of models), had a significant relationship with PTD rates.
Unlike LBW, race and ethnic characteristics influence PTD rates across the study area in different ways. Whereas LBW rates are driven up in counties with low/medium Hispanic populations compared to high percentage counties, PTD is more strongly associated with higher percentages of Black populations. Further, population structure and socioeconomic status indicators provide the most robust indication of counties more likely to have higher PTD than the national average. Although no consistent indictor of PTD was discovered across all models (years), higher rates were more heavily influenced by low and medium gross rent across many years (models).
Many more indicators were influential in decreasing the likelihood of PTD across the study area (Figure 1B). Here, like in the LWB analysis, counties with low and moderate Black populations are significantly less likely to have PTD than counties with high black populations. As expected, counties with low percent females, female-headed households, female labor force participation had a decreased likelihood of high PTD rates in comparison to counties with high percentages of these populations. However, the influence was not standard across all years. Finally, random positive (decreasing) influence on several years of PTD was found for counties with low and medium extractive industry employment, per capita income, renters, nursing home residents, and English language proficiency compared to counties with high percentages indicators. Here, access and functional needs indicators were more influential in the earlier years (2000 – 2005) than in later years, indicating the presence of possible PTD related interventions for these groups in later years.