Darrow et al. weight in each month of pregnancy, control- ling for concentrations of the same pollut- ant in the other months of pregnancy. We scaled month-specific effect estimates to IQR increases in 4-week pollutant concentra- tions as shown in Table 2. Overall, results were suggestive of air pollutant concentra- tions in the ninth month of pregnancy being associated with reductions in birth weight, although only the association for NO2 was statistically significant. We used monthly esti- mates to estimate the cumulative effect of an IQR increase in pollutant concentrations across all months of pregnancy as shown in the upper right-hand corner of each graph in Figure 1. An IQR increase in NO2 concentra- tions during the 9-month pregnancy period (5 ppb) was associated with a 9.2-g decrease in birth weight; an IQR increase in O3 in all months of pregnancy (23 ppb) was associ- ated with a 30.8-g decrease in birth weight; and an IQR increase in PM2.5 organic car- bon in all months of pregnancy (1.2 µg/m3) was associated with a 17.1-g decrease in birth weight. Unconstrained distributed lag results (i.e., without forcing a cubic polynomial shape across months) were less smooth across months but overall showed similar patterns. Table 4 shows the results from mod- els assessing effect modification by race and CO (1-hr) –2.5 (–9.1 to 4.0) NO2 (1-hr) –9.2 (–14.0 to –4.5) SO2 (1-hr) 2.1 (–5.1 to 9.3) –5 –10 5 0 5 0 5 0 –5 –10 –5 –10 20 10 0 –10 –20 –5 –10 5 0 5 0 5 0 –5 –10 –5 –10 20 10 0 –10 –20 1 2 3 4 5 6 7 8 9 Month of pregnancy PM2.5–10 (24-hr) –6.0 (–18.1 to 6.0) 1 2 3 4 5 6 7 8 9 Month of pregnancy PM2.5 elemental carbon (24-hr) –0.3 (–12.6 to 11.9) 1 2 3 4 5 6 7 8 9 Month of pregnancy PM2.5 organic carbon (24-hr) –17.1 (–27.4 to –6.9) 1 2 3 4 5 6 7 8 9 Month of pregnancy PM2.5 nitrate (24-hr) 14.2 (–29.2 to 57.6) 1 2 3 4 5 6 7 8 9 Month of pregnancy PM2.5 sulfate (24-hr) –28.7 (–104.0 to 46.5) 1 2 3 4 5 6 7 8 9 Month of pregnancy PM2.5 water-soluble metals (24-hr) –29.7 (–62.3 to 2.9) t 5 n a IrrseapRGQm itllrspeaecoun –50 n i –10 t 5 n a IrrspeaRQGm itllrsenpauoec –50 n i –10 t 5 n a IrrspeaRGQm itllrsenpaecou –50 n i –10 20 t IRQ tlluan 10 rrspeaGm irsenpaoec–100 in–20 1 2 3 4 5 6 7 8 9 Month of pregnancy 1 2 3 4 5 6 7 8 9 Month of pregnancy 1 2 3 4 5 6 7 8 9 Month of pregnancy O3 (8-hr) –30.8 (–44.8 to –16.9) PM10 (24-hr) –6.0 (–19.8 to 7.9) PM2.5 (24-hr) 7.6 (–9.0 to 24.2) ethnicity for pollutant concentrations in the third trimester. The pollution × race interac- tion terms were statistically significant for CO, O3, PM10, PM2.5, and PM2.5–10. Overall, we observed more statistically significant associa- tions between air pollutants and birth weight in the Hispanic and non-Hispanic black groups than in the non-Hispanic white group. Notably, the association between PM2.5 water- soluble metals and birth weight was statisti- cally significant and of similar magnitude for all three racial and ethnic groups. Capture-area analysis. Table 5 presents the change in birth weight and 95% confi- dence intervals per IQR increase in pollutant concentrations for the population of pregnan- cies within 4 miles of a monitor. We observed associations between third-trimester PM2.5–10 and increased birth weight and between CO concentrations in the first month of gesta- tion and decreased birth weight. Although confidence intervals were wide and largely compatible with the estimates observed in the five-county analysis, these estimates provided little additional support for the associations observed in five-county population. Discussion We investigated relationships between ambi- ent concentrations of 12 pollutants during two a priori gestational windows of interest and birth weight in full-term infants. In the five- county analysis, we observed little evidence of an association between air pollution con- centrations in the first month of gestation and reduced birth weight; we observed one asso- ciation between PM2.5 nitrate and increased birth weight. Third-trimester concentrations of four pollutants (NO2, SO2, PM2.5 elemen- tal carbon, and PM2.5 water-soluble metals) were associated with small decreases in birth weight. The capture-area analyses, which used measurements close to the maternal residence, provided little support for the associations observed in the five-county analysis, although confidence intervals were wide. The capture- area analyses did show an association between CO levels in the first month of gestation and decreased birth weight. Given the number of statistical tests con- ducted in this analysis and the potential for type 1 errors, results should be interpreted in the context of previous research. Previous studies have reported associations between traffic-related pollutants and measures of fetal growth such as birth weight and small for gestational age (Aguilera et al. 2009; Bell et al. 2007; Gouveia et al. 2004; Liu et al. 2003; Wilhelm and Ritz 2003, 2005). Our observed associations between third-trimester NO2 and PM2.5 elemental carbon concentrations and reduced birth weight provide some additional evidence of this relationship. However, the specific pollutants and gestational windows 1 2 3 4 5 6 7 8 9 Month of pregnancy 1 2 3 4 5 6 7 8 9 Month of pregnancy 1 2 3 4 5 6 7 8 9 Month of pregnancy Figure 1. Associations between ambient pollutant concentrations and birth weight distributed across the months of pregnancy. The estimated change in birth weight and the 95% confidence interval displayed in the upper right-hand corner of each plot correspond to an IQR increase in the cumulative air pollution concentration during the entire pregnancy period. Estimates and 95% confidence intervals are presented graphically for the month-specific effects. 734 ovlume 119 | number 5 | May 2011 • Environmental Health Perspectives