Int. J. Environ. Res. Public Health 2012, 9 275 potential confounders mentioned above. The different exposure windows we studied were; the first and second trimesters of pregnancy, and the last week of pregnancy. We estimated 95% confidence intervals (CI) for all association parameters (slopes and odds ratios). All analyses were made using R statistical software package (2.13.0) [20]. 3. Results About one half of the mothers were primiparous and about a third gave birth to their second child. 5.3% of the births were preterm. The distribution of duration of gestation was skewed towards the lower end of the spectrum (Figure 1). The variability in birth weight was greater among gestations with longer durations (Figure 2). Figure 1. Histogram of duration of gestation. The average pollution levels and meteorology were relatively similar for different time frames of pregnancy (Table 1). There were moderate negative correlations between O3 and NO2 (Table 2), and positive correlations between O3 and ambient temperature. O3 and relative humidity were strongly negatively correlated. NO2 and ambient temperature had a moderate negative correlation. NO2 and relative humidity was positively correlated. Both O3 and NO2 had a slight positive correlation between first and second trimester averages. The correlation between the first trimester and the last week of gestation averages was negative for both pollutants. The correlation between the second trimester O3 and the last week of gestation O3 averages was negative. There was no correlation between the second trimester NO2 and the last week of gestation NO2 averages.