NO2 is mainly a secondary pollutant (formed from the oxidation of NO primary emissions) and that LUR estimates of BTEX capture the influence of additional traffic emission sources such as parking lots, our results suggest that BTEX could be a more specific marker for exhaust toxins of concern for pregnancy in studies conducted within urban areas. Conclusions We found an effect of exposure to traffic-re- lated air pollutants (BTEX and, to a lesser extent, NO2) on birth weight among pregnant women who live in an urban area and spent < 2 hr/day in nonresidential outdoor loca- tions. Although the magnitude of the asso- ciation was higher for BTEX, the independent effect of different air pollutants with common emission sources remains to be determined. When possible, time–activity patterns during pregnancy should be considered to examine whether they may affect exposure misclassifi- cation. 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