Impact of ambient air pollution on birth outcomes : systematic review of the current evidences

Background: There is growing interest in the possible association between maternal exposure to air pollutants and reproductive outcomes, particularly birth weight and gestational duration. Four systematic reviews of data were published in 2004-2005, but the wide variability of methods and results among the different studies produced conflicting conclusions.This study was done to establish whether recent literature has provided more conclusive evidence regarding a link between air pollutants and birth outcomes. Methods: We reviewed 18 original epidemiological studies on maternal exposure to particulate matter (PM), NO2, CO and O3, and outcomes of preterm delivery or low birth weight published since 2004. Results: Large variability across studies in design, precision in maternal georeferentiation, methods in exposure assessment, and type of pollutant considered, limited the strength of the evidence of adverse affects of ambient air pollution on birth outcomes. Nevertheless, evidence suggests exposure to particulate matter, especially at its finest fraction (PM2.5), may have the potential to adversely affect birth weight. We further found limited evidence of a possible association between maternal exposure to air pollutants during the first trimester and increased risk of preterm delivery. Discussion: The observed adverse effects were generally small. However, possible important factors such as maternal activity pattern, diet, smoking and occupation, that are usually not reported on the birth certificate, might have led to exposure misclassification and confounding and could have hidden moderately increased risks. In conclusion, additional studies since 2004 have not been able to conclusively show a definitive correlation between air pollution and adverse birth outcomes; although it appears that small size particulate matter could affect birth weight. Additional well-conducted studies that include detailed information on maternal risk factors and using validated models for estimating maternal exposure are needed to establish the extent of the association between air pollution and birth outcomes.