BMC Public Health 2009, 9:237 http://www.biomedcentral.com/1471-2458/9/237 Background A growing number of studies have reported associations between area-based characteristics and perinatal health. Research has focussed on neighbourhood economic dep- rivation in relation to gestational age [1-8] and birth- weight [5,7,9-14]. Neighbourhood deprivation has been found to have an unfavourable association with these birth outcomes, even accounting for individual socio-eco- nomic status. Some studies have focussed on neighbour- hood segregation and gestational age [15-19] or birthweight [15,16,19-21], but the evidence is conflicting. Studies demonstrate both protective [16,19] and unfa- vourable [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 stud- ies 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 con- trast to area poverty measures which reflect mean income [24]. Although controversial, income inequality is recog- nised 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 path- way 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 ine- quality 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-gesta- tional-age (SGA) birth. PTB is responsible for a large pro- portion 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. Methods Data Data were obtained from the 1) Québec birth file and 2) 2001 Canada census. Singleton births for a 5-year period (1999–2003) centered on the 2001 Census year were extracted (N = 356,147). Residential area at time of birth was specified as administrative local community service center (CLSC) districts (N = 166). CLSCs are areas of health service delivery [33] where variability in area-dep- rivation has been previously documented [34]. CLSCs are comparable to neighbourhoods as their borders are based on historically defined neighbourhoods, integrate geo- graphic or natural boundaries, and their residents report a subjective sense of place [35]. We examined a fused ver- sion of CLSCs provided by the health ministry (N = 147) to ensure areas were sufficiently large for the calculation of income inequality [mean population 51,564 (range 3,700 to 142,025); mean births 2,469 (range 230 to 8,674)]. Four northern CLSCs (representing 3,027 births) were excluded because reliable census data were not available for these areas. The final dataset was hierarchically struc- tured with 353,120 births nested in 143 CLSCs. Page 2 of 11 (page number not for citation purposes)