Int. J. Environ. Res. Public Health 2012, 9 273 NO2 exposure estimates and studied outcomes. The effect of first trimester ozone exposure, known to cause oxidative stress, was smallest among women who conceived during autumn when vitamin D status, important for fetal health, in Scandinavian women is the highest. Keywords: pregnancy; preterm birth; vehicle emissions; ozone; nitrogen dioxide 1. Background There is an ever growing body of evidence that ambient air pollution levels during gestation have adverse effects on birth outcomes [1-12]. Maternal exposure to carbon monoxide (CO), or motor vehicle exhaust, late in the pregnancy was associated with preterm birth in Vancouver, Canada and in California [4-6]. Late pregnancy exposure to fine and respirable particulate matter (PM2.5, PM10) has also been associated with an increased risk of preterm delivery in California [5,6]. Early pregnancy exposure to ozone (O3) and PM10 increased the risk of preterm delivery in Brisbane, Australia [13]. Increased concentration of several pollutants during the first and third trimester were associated with preterm birth in the Republic of Korea [9]. A study from the Czech Republic, where sulphur dioxide levels (SO2) were rather high, found SO2 to be associated with preterm birth regardless of period of exposure [12]. Additionally, exposure to particles during early gestation as well as exposure to NOx during early or late gestation was associated with preterm birth. It has been suggested that there may be some seasonality in the effects from exposure to air pollution during different exposure windows of gestation on the duration of gestation [10]. However, there is no unambiguous answer as to which specific elements of pollution or during which period of pregnancy, has an impact on pregnancy outcomes. Sram et al. conclude that the evidence is strong enough to infer causality of air pollution on low birth weight, LBW (birth weight < 2.5 kg), while the evidence of association between air pollution and preterm birth is deemed insufficient to infer causality [2]. More recently, Bosetti et al. reviewed a variety of studies and still found that the studies on maternal exposure to respirable particles during pregnancy do not provide conclusive evidence of an association with the risk of preterm birth and low birth weight [14]. Other authors conclude that for traffic related pollution, the association with an increased risk of preterm birth seems to be consistent [7]. However, several recent studies were negative: in the Dutch PIAMA birth cohort there was no significant association between preterm birth and NO2 during first trimester, last month or entire pregnancy [15]. Neither was there any significant association between NO2 and preterm birth in a study from California [3]. From Sydney an unexplained protective effect of NO2 has been reported [10] and in Valencia the dose-response relationship was V-shaped [16]. The few studies of O3 have not found robust associations [3,4,10,13]. Preterm birth is a risk factor not only in early life, but is also associated with respiratory and cardiovascular disease later in life [17,18]. Thus more and large studies are needed, especially from Europe where most of the reported studies of air pollution exposure have been small. Studies of maternal air pollution exposure and preterm birth/LBW build on different types of exposure variables; concentrations at monitoring stations, modelled concentrations or proximity to