The joint influence of area income, income inequality, and immigrant density on adverse birth outcomes: a population-based study

A growing number of studies have reported associations between area-based characteristics and perinatal health. Research has focussed on neighbourhood economic deprivation in relation to gestational age [1‐8] and birthweight [5, 7, 9‐14]. Neighbourhood deprivation has been found to have an unfavourable association with these birth outcomes, even accounting for individual socio-economic status. Some studies have focussed on neighbourhood segregation and gestational age [15‐19] or birthweight [15, 16, 19‐21], but the evidence is conflicting. Studies demonstrate both protective [16, 19] and unfavourable [15‐18, 20‐22] associations with birth outcomes.

Recent work has evaluated associations between area characteristics and birth outcomes across different social classes, with particular attention on ethnic segregation and racial groups in the US [8, 15, 17‐21]. The evidence suggests that African-American and white mothers may be differentially susceptible to segregation, although study results are not consistent [17, 20]. A small number of studies have found that area deprivation also interacts with race [5, 6, 8, 20].

While such research attests to a contextual influence that varies among US racial classes, information for other nations and social markers is lacking. In other settings, measures other than race alone may better represent social differences. Whether the influence of area characteristics varies according to such factors has not been evaluated. Some evidence suggests that the influence of area-based economic deprivation may vary depending on maternal foreign-birth status [7].

These knowledge gaps also apply to area characteristics emerging as predictors of birth outcomes, such as income inequality. Measures of income inequality represent the unequal distribution of income within areas [23], in contrast to area poverty measures which reflect mean income [24]. Although controversial, income inequality is recognised as a determinant of mortality [25], and, possibly, of birth outcomes [23, 26]. Studies have focussed on state or country-level income inequality, rather than small areas where evidence has been less compelling [27]. However, an adverse influence of income inequality on preterm birth (PTB) was reported in a recent study of US counties [23]. The investigators found that the influence of income inequality depended on race [23]. A separate study on cumulative exposure to income inequality reported an association with PTB for Hispanic but not Black or White ethnicity [28]. No study outside the US has yet examined birth outcomes and income inequality.

The study of place-based influences is also challenged by the complex relationships between contextual factors and health outcomes [29]. Income inequality might be a pathway through which area income exerts its effects [24], or might independently influence birth outcomes. One way to consider these issues is to examine the joint influence of different area characteristics [26]. Area deprivation and segregation have been investigated together in a limited number of studies [5, 17, 20]. No study has considered income inequality together with other area variables in relation to birth outcomes.

These gaps in the literature are important to address as public health interventions often focus on sub-groups at risk, an approach that may be justified by the limited resources available for interventions. An important first challenge is to identify the appropriate sub-groups, and then act to reduce the inequalities present within these groups.

The objective of this study was to examine the association between birth outcomes and area income, income inequality and segregation across different social markers in Québec, Canada. Québec is a province with a supportive social welfare system and universal health care [7], and can be considered more egalitarian than US states [30]. The outcomes analysed were PTB and small-for-gestational-age (SGA) birth. PTB is responsible for a large proportion of infant morbidity and mortality [6]. SGA birth is useful as measure of fetal growth restriction [31, 32]. We did not consider low birth weight [32]. We evaluated whether the relationship between area characteristics and these birth outcomes was modified by two social markers, maternal birthplace and education.

This study examines, in a Canadian setting, associations between area income, immigrant density, and income inequality in relation to PTB and SGA birth. Previous studies have not considered the joint influence of these neighbourhood characteristics. We build on and extend previous research documenting interactions between area context and race [5, 6, 8, 15, 17, 18, 20, 21, 23]. Our results indicate that, independent of individual maternal characteristics, the influence of area context varies according to different social markers.

Our results are consistent with studies demonstrating an adverse influence of high area deprivation on PTB [1‐6, 8] and SGA birth [3‐5]. Though area poverty was in our study more strongly associated with SGA birth than PTB, this finding was not observed in another study evaluating these same two birth outcomes [5]. The literature suggests PTB may be related to stress, possibly to a greater extent than SGA birth [44, 45], while SGA birth may be more strongly related to lifestyle factors such as tobacco use [46, 47]. Thus, the stronger associations with SGA birth may be related to the lower material/social resources present in deprived neighbourhoods, which may predispose less favourable behaviours among mothers. Our stratified analyses indicated that area poverty was more strongly associated with SGA birth among mothers with low compared to high education, this finding possibly being due to a greater level of unfavourable health-related behaviours in socio-economically vulnerable populations [48]. Our stratified analyses also indicated that area poverty was associated with PTB and SGA birth among Canadian-born but not foreign-born mothers. A separate study reported an association between area income and both PTB and full-term low birthweight (a proxy for SGA birth) among long-term residents but not recent immigrants of Toronto, Canada [7], in line with our findings. Foreign-born mothers, potentially healthier because of the selective process of immigration [7, 49], may be more resilient than native-born populations to area deprivation. Other studies evaluating the influence of area disadvantage according to race (not maternal birth place) have not yielded consistent results [5, 6].

We found a protective association between high immigrant density and PTB, and the magnitude of the association was greater than for area poverty. This finding is in line with research documenting fewer adverse mental health outcomes in areas with higher minority populations [50]. Stratified analyses, however, showed that high immigrant density was protective for PTB among Canadian-born mothers, but unfavourably associated with PTB among foreign-born mothers. This intriguing finding has not been previously reported. A study restricted to US Black mothers did, in comparison, find an elevated odds of PTB for racially concentrated neighbourhoods among mothers that were US-born but not those foreign-born [15]. The authors did not, however, account for neighbourhood income. Hence it is difficult to know the extent to which racial concentration may be a marker for poverty in the study. What can explain the interaction observed in our study? The stronger association with PTB than with SGA birth suggests that immigrant density may be operating through stress mechanisms, assuming PTB is more strongly linked to stress [5]. Thus, high immigrant density may be associated with conditions that reduce stress in native-born mothers, but increase stress in foreign-born mothers. Such conditions might arise if, for example, employment opportunities were greater for native- than for foreign-born individuals due to prejudice or network integration. The immigrant composition of Québec, with ties to French-speaking source countries, might also be driving these results although the underlying pathways are unclear.

We also found high immigrant density was associated with greater odds of PTB among mothers with high but not low education. Educated mothers might more readily access benefits conferred by high immigrant density (e.g., employment opportunities). Other studies, mainly focussing on segregation, have considered racial class but not education as a modifier of neighbourhood effects. Maternal education is generally treated as a control variable and not as a potential modifier [17, 18, 20]. Studies are, however, unclear on whether ethnic segregation is protective [16, 19] or harmful [5, 15‐18] for PTB. An issue that complicates comparisons of studies is the wide spectrum of segregation measures that have been studied. However, when we compare our results to other studies that used a percentage-based indicator for segregation, results are still contradictory [5, 15, 17]. Thus, immigrant density in Québec may not be comparable to racial segregation in the United States.

While non-stratified models suggested that immigrant density may be protective for PTB, its associations with SGA birth were unfavourable. Our findings are in line with research supporting an adverse influence of ethnic segregation on SGA birth [5, 16, 22]. Unlike PTB, however, stratified analyses did not support a differential influence according to education.

Evidence for a contextual influence of income inequality was weaker than for area poverty and immigrant density, in line with research showing income inequality has a weaker influence in small areas compared to larger places [27]. Nevertheless, our results suggested an unexpectedly protective association, somewhat more for SGA birth than PTB. Protective associations between income inequality and adult outcomes have been observed by others [51‐54], but not for birth outcomes. The sparse research performed thus far on birth outcomes supports an adverse association between high income inequality and PTB [23, 28] or low birthweight [26]. Income inequality in Québec may be a characteristic of affluent neighbourhoods, affluence itself being associated with favourable birth outcomes [54]. Income inequality could also be a proxy for other unrelated neighbourhood factors favourably associated with birth outcomes. We used the CV, an indicator sensitive to the upper ends of the income spectrum [55], and associations could be different with indicators sensitive to other parts of the income spectrum. Two competing theories in the income inequality literature may also be considered [23, 56]. These include the "neo-materialist" pathway which posits that underinvestment in material capital in less equal communities translates into lost opportunities and possible adoption of harmful health behaviours, and the "psychosocial" pathway which posits that relative comparisons experienced by less well-off individuals in unequal communities causes stress resulting in biologic effects [23, 56]. These mechanisms suggest affluent neighbourhoods with high income inequality might have higher material capital (i.e., "neo-materialist" pathway) that would be expected to be beneficial for birth outcomes. Alternatively, neighbourhoods favouring environments in which striving for better life conditions is perceived positively (i.e., psychosocial pathway) may also be beneficial for birth outcomes.

Our results raise the possibility that the influence of income inequality on SGA birth might be more protective among mothers with high education. The protective influence of income inequality may more readily translate into health benefits when socio-economic status is high. The only comparable study evaluating the influence of income inequality according to class found an adverse influence on PTB among non-White mothers in the United States [23]. Other studies examining effect modification have focussed on adult outcomes [57‐59].

This study is subject to limitations. We could not account for tobacco use and income which may confound or mediate relationships, although such factors do not fully account for area effects [1]. Statistical power was lower in stratified analyses for foreign-born mothers, due to smaller sample size. Our contextual indicators may be proxies for underlying factors that we cannot pinpoint with certitude. Although we examined several attributes of areas, we did not examine other dimensions such as segregation that could also contribute to adverse birth outcomes. Our indicator of income inequality was calculated from census data, which may not be as accurate as tax data [60]. We do not know length of residence in neighbourhoods, and the selection of mothers into neighbourhoods based on individual characteristics cannot be ruled out. The administrative boundaries that we used might not be the best level for the analysis of contextual effects [35]. Research indicates that the use of larger areas such as CLSCs might attenuate measures of contextual effects, however, these areas facilitate the study of income inequality [61]. Last, the extent to which results are generalizable to other non-US settings is unknown.

This study demonstrates that the association between area factors and adverse birth outcomes in Québec is modified by maternal birthplace and education. Some studies have found race interacts in a similar manner, and our results indicate that social class indicators other than race influence this relationship. While both individual and area-level characteristics need to be accounted for in public health initiatives addressing adverse birth outcomes, there may be a further need to consider the interaction between these factors.