This article is a commentary on a study published in the journal Pediatrics 2005;115(1):121-128. There is growing interest in the role of environmental factors in pregnancy and its outcome. The results of several epidemiological studies suggest that maternal exposure to air pollutants during pregnancy may have an impact on pregnancy, particularly by increasing the risk of congenital malformations, low birth weight (< 2,500 g), and preterm birth. The objective of this study was to analyze the relationships between monitoring data on atmospheric concentrations of PM2.5 and carbon monoxide and pregnancy outcomes, particularly low birth weight for gestational age, using data from birth certificates of more than 18,000 children born in 2000 in California. Exposure indicators were constructed over several time periods to identify a potential critical window of exposure. The analysis was limited to singleton births at 40 weeks of gestation in order, on the one hand, to calculate maternal exposure over periods of identical duration for all births and, on the other hand, to specifically examine the impact of pollutants on intrauterine growth, while controlling for the effect of gestational age on birth weight. This study, which involves a large sample of births, supports the findings of other research suggesting that exposure to urban air pollutants has a significant impact on intrauterine growth, even though the respective impacts of different pollutants, the mechanisms of action, and the existence of a potential critical exposure window remain highly debated. The failure to account for certain important risk factors for low birth weight prevents a conclusion regarding the causal nature of the observed relationship. This warrants further investigation through new studies.