BMC Public Health 2009, 9:237 http://www.biomedcentral.com/1471-2458/9/237 way 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, neighbour- hoods 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 influ- ence 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 seg- regation that could also contribute to adverse birth out- comes. 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 neighbour- hoods, 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 ine- quality [61]. Last, the extent to which results are general- izable to other non-US settings is unknown. Conclusion 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 influ- ence 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. Abbreviations CLSC: local community service center; CV: coefficient of variation; GEE: generalized estimating equations; PTB: preterm birth; SGA: small-for-gestational age. Competing interests The authors declare that they have no competing interests. Authors' contributions designed the study. and JG reviewed the literature. guided and JG performed the data analyses. and JG interpreted the results and wrote the manuscript. MD pro- vided structured neighbourhood-level data, helped inter- pret results and revised the manuscript for important intellectual content. All authors read and approved the final version of the manuscript. Acknowledgements This study was not funded. MD is supported by a University of South Aus- tralia Research Chair for Social Epidemiology. 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