The current study was based on birth certificates and had the advantage of using a large number of birth records, thus reducing uncertainties due to random error misclassification and selection bias more common in studies with a small sample size. A limitation of this type of study was that birth record studies are typically restricted to routinely recorded information and may not fully control for confounding from individual maternal or fetal risk factors such as maternal passive/active smoking, alcohol consump- tion, infections and diabetes. A research by Ritz and Wilhelm (2008) indicated that for adverse pregnancy outcomes, this may not be a cause of concern. They found that trimester-specific association estimates for air pollution did not appear to be confounded by covariates compared to those routinely collected on birth certificates. In the current study, the ward-based socio- economic status ranking that characterized the ward's population was added to the routinely collected covariates. The ward-based socioeconomic status ranking represents several individual risk factors and possible confounders. Statistical wards are designed a- priori to include relatively homogeneous populations with respect to ethnicity, lifestyle and social class (Central Bureau of Statistics,
2011). The ward-based socioeconomic status ranking was not found to be correlated with maternal exposure to air pollution.