Darrow et al. in non-Hispanic whites. In Connecticut and Massachusetts, associations between PM2.5 concentrations during pregnancy and birth weight were stronger in black women com- pared with white women (Bell et al. 2007). Another study in the northeastern United States also reported a stronger association between CO concentrations during pregnancy and low birth weight in African Americans (Maisonet et al. 2001). Differential suscepti- bility could be due to increased personal expo- sure to ambient air pollution in certain racial and ethnic groups (e.g., access to air condi- tioning) or differential biological sensitivity to air pollution stemming from differences in underlying health status, access to health care, or psychosocial stress (O’Neill et al. 2003). More research is needed to investigate poten- tial differences in susceptibility among racial and ethnic population subgroups. A major challenge in the field of air pollu- tion and pregnancy outcomes has been iden- tifying gestational windows of vulnerability. Because the correct gestational window(s) of exposure are unknown, we conducted distrib- uted lag analyses, a data-based approach that may help to identify vulnerable gestational windows, particularly when exposures are cor- related across the gestational windows. This approach has not previously been applied in the context of air pollution and pregnancy out- comes to our knowledge. Results of our analy- ses suggested that late pregnancy, specifically the ninth month, is a gestational window of vulnerability to air pollution. Evidence of early pregnancy as a vulnerable gestational window for air pollution effects on birth weight was weaker, with the exception of an association with first-month CO concentrations observed in the capture-area analysis. This could be a consequence of our inability to identify all conceptions, because we could identify only fetuses who survived to 20 weeks. If exposure to air pollution in early pregnancy increases the risk of fetal loss in addition to restricting fetal growth, associations between air pollution and birth weight would be underestimated. It is also possible that air pollution exposures influence fetal growth throughout gestation, but we were able to detect associations with birth weight only for exposures in the end of gestation because there is limited opportu- nity for catch-up growth between late expo- sure windows and birth. Studies that examine repeated measures of fetal growth throughout pregnancy rather than birth weight overcome this problem. Measurement error in the ambient air pol- lution concentrations is a limitation of our study as well as previous studies in this field. In the temporal analytic setting, this error is attributable to both instrument imprecision and spatial variability, which are expected to attenuate associations (Goldman et al. 2009; 736 Zeger et al. 2000). This might explain some of the results we observed but would be unlikely to induce spurious associations. Concerns about exposure measurement error motivated our capture-area analyses, in which mothers were assigned pollutant concentra- tions measured within 4 miles of their resi- dential address. However, with the exception of first-month CO, observed associations were not stronger in the capture-area analyses than in the five-county analysis, and point esti- mates provided little support for associations between local air pollutant concentrations and reduced birth weight. These findings do not strengthen conclusions regarding the harm- ful effect of air pollutant concentrations in Atlanta on birth weight suggested by the five- county analysis. However, it is possible that the local ambient monitors did not provide a better exposure measure for the capture- area population. Spatial variability of pollut- ant concentrations can be substantial even within 4 miles for vehicle emissions such as CO, NOx, and PM2.5 elemental carbon. Depending on the location of the monitor and the degree to which it is affected by local sources versus regional background, residen- tial proximity to a monitor does not guaran- tee that air quality observations at that site are a better exposure measure. Pregnant women may also spend a large portion of their day away from their residence, and with approx- imately 20% of women in Atlanta chang- ing residences during pregnancy, exposure assignment based on the residence at birth is problematic for assessment of early gestational windows (Miller et al. 2010). In all of the analyses presented here, we made comparisons across time to reduce the plausibility of confounding by individual- level risk factors, which are unlikely to be associated with short-term fluctuations in air pollution (Strickland et al. 2009). This approach complements the existing literature, which is primarily based on spatiotemporal contrasts of exposure. Although we controlled for long-term temporal trends and seasonal trends and assessed confounding by meteoro- logical variables, it is possible that results were confounded by other unmeasured risk factors that are associated with temporal fluctuations in air pollutant concentrations. Conclusion We observed associations between air pol- lutant concentrations in late pregnancy and small reductions in birth weight in full-term infants. However, conclusions are somewhat tempered by the lack of support for these associations when air quality concentrations measured closer to the maternal residence were used. Our assessment of PM2.5 chem- ical components suggests that associations between PM and pregnancy outcomes may vary by chemical composition of the particles. Observed associations between birth weight and traffic-related pollutants (i.e., NO2 and PM2.5 elemental carbon) during the final trimester of pregnancy suggest that traffic sources of pollution should be further investi- gated in relation to fetal growth. Ref Rencs Aguilera I, Guxens M, Garcia-Esteban R, Corbella T, Nieuwenhuijsen MJ, Foradada CM, et al. 2009. Association between GIS-based exposure to urban air pollution during pregnancy and birth weight in the INMA Sabadell cohort. Environ Health Perspect 117:1322–1327. Bell ML, Ebisu K, Belanger K. 2007. Ambient air pollution and low birth weight in Connecticut and Massachusetts. Environ Health Perspect 115:1118–1124. Butler AJ, Andrew MS, Russell AG. 2003. 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