Numerous studies have reported that traffic-related air pollution is associated with intrauterine growth restriction, low birth weight, and preterm birth, but studies to determine the most harmful pollutants and the most susceptible periods for exposure have been inconclusive. Aguilera et al. studied exposure to nitrogen dioxide (NO2) and aromatic hydrocarbons in association with fetal growth among 562 pregnant women in the Spanish INMA Sabadell study cohort. Exposure to aromatic hydrocarbons during early pregnancy was inversely associated with growth in biparietal diameter during weeks 20