We observed strong and consistent associations for the second trimester and the end of pregnancy (which coincide for early preterm births). This collection of results may indicate that exposures to pollutants nearer parturition may be contributing. Although the specific pathways need to be further clarified, inflammation has been hypothesized as a potential mechanism of action for preterm birth [24]. Inflammation may reflect early activation of the normal parturition cascade, in which proinflammatory mediators such as cytokines are typically induced. These pollutants may result in inflammatory responses that cause preterm birth. Ongoing studies may soon be able elucidate this potential mechanism.