Low birth weight (LBW; birth weight < 2,500 g) is a predictor of infant morbidity and mortality. The evidence for associations between exposure to airborne particulate matter (PM) and LBW is growing ( Fleischer et al. 2014 ; Pedersen et al. 2013a ; Stieb et al. 2012 ). Recent meta-analyses show heterogeneity across studies conducted in different areas ( Dadvand et al. 2013 ; Sapkota et al. 2012 ; Stieb et al. 2012 ). Many of these studies have also examined continuous birth weight, and some point toward an association between PM and mean birth weight adjusted for gestational age ( Pedersen et al. 2013a ; Stieb et al. 2012 ).