This article is a commentary on the following study: "Estimated Relation between Ambient Air Quality and Selected Birth Outcomes, Seven County Study, 1997–2000," by Gilboa SM, Mendola P, Olshan AF, Langlois PH, Savitz DA, Loomis D, Herring AH, and Fixler DE.Am J Epidemiol 2005;162:238-252.