Int. J. Environ. Res. Public Health 2012, 9, 272-285; doi:10.3390/ijerph9010272 OPEN ACCESS International Journal of Environmental Research and Public Health ISSN 1660-4601 www.mdpi.com/journal/ijerph Article Temporal Variation in Air Pollution Concentrations and Preterm Birth—A Population Based Epidemiological Study David Olsson 1,*, Magnus Ekström 2 and Bertil Forsberg 1 1 2 Division of Occupational and Environmental Medicine, Department of Public Health and Clinical Medicine, Umeå University, 901 87 Umeå, Sweden; E-Mail: bertil.forsberg@envmed.umu.se Centre of Biostochastics, Swedish University of Agricultural Sciences, 901 83 Umeå, Sweden; E-Mail: magnus.ekstrom@slu.se * Author to whom correspondence should be addressed; E-Mail: david.olsson@envmed.umu.se; Tel.: +46-90-785-3479; Fax: +46-90-786-9995. Received: 16 December 2011; in revised form: 9 January 2012 / Accepted: 13 January 2012 / Published: 18 January 2012 Abstract: There is growing evidence of adverse birth outcomes due to exposure to air pollution during gestation. However, recent negative studies are also reported. The aim of this study was to assess the effect of ozone and vehicle exhaust exposure (NO2) on the length of the gestational period and risk of preterm delivery. We used data from the Swedish Medical Birth Registry on all vaginally delivered singleton births in the Greater Stockholm area who were conceived during 1987–1995 (n = 115,588). Daily average levels of NO2 (from three measuring stations) and ozone (two stations) were used to estimate trimester and last week of gestation average exposures. Linear regression models were used to assess the association between the two air pollutants and three exposure windows, while logistic regression models were used when analyzing associations with preterm delivery (<37 weeks gestation). Five percent were born preterm. The median gestational period was 40 weeks. Higher levels of ozone during the first trimester were associated with shorter gestation as well as with an elevated risk of preterm delivery, the odds ratio from the most complex model was 1.06 (95% CI: 1.00–1.13) per 10 μg/m3 increase in the mean daily 8-h maximum concentration. Higher levels of ozone during the second trimester were associated with shorter gestation but the elevated risk of preterm delivery was not statistically significant. Higher levels of ozone and NO2 during the last week of gestation were associated with a shorter duration of gestation and NO2 also with preterm delivery. There were no significant associations between first and second trimester