Int. J. Environ. Res. Public Health 2012, 9 281 patterns could confound the associations driven by studying temporal fluctuations in air pollution concentrations. The only criterion for a child to be entered into the study was to be a vaginally delivered singleton birth during the study period; this means that we have no restriction as to how many times one woman is represented. A consequence of this is that the observations are not independent, and there may be some individual factors that strongly predicts duration of gestation, for example some genetic predisposition. However, the direction and magnitude of the associations did not change when restricting the models to primiparous women only (Appendix A). A typical problem in air pollution studies is potential confounding, and in studies of birth outcome socioeconomic status, lifestyle, country of origin etc. may differ between areas with more or less air pollution. As we only study the influence of temporal variability in the air pollutants, we primarily need to worry about potential confounding for variables that vary in time in a similar way [11,24]. We did not observe any confounding effect on the results for NO2. We observed some confounding from the meteorological factors and time of conception on the results for first trimester O3, inflating the ORs from 1.06 per IQR increase in the unadjusted model to 1.16 per IQR increase in the adjusted model. Season of conception, temperature and relative humidity could be good proxies for sunlight exposure, which is correlated with ground level O3 formation. Sunlight is associated with maternal vitamin D status. Maternal vitamin D levels are important for fetal health, and vitamin D deficiency during pregnancy has been associated with preeclampsia [25]. Inflammation in early pregnancy is associated with an increased risk of preterm delivery [26]. Vitamin D deficiency is associated with increased circulating inflammatory proteins, and vitamin D status seems to modulate C-reactive protein levels [27]. In a recent study exposures to PM and O3, but not NO2, were associated with increased C-reactive protein concentrations in early pregnancy [28]. Vitamin D status has a clear seasonal variation in Scandinavian women, and is best in the autumn [29]. In this study the effect of first trimester O3 levels was smallest among women who conceived during the autumn. One of the limitations in this study is the lack of adjustment for socioeconomic factors. There are no good proxies for socioeconomic status in the Medical Birth Registry. Socioeconomic status varies more strongly in space than in time, at least in a shorter time span. It might be plausible that there is confounding from temporal variations in socioeconomic status on the associations between air pollution and preterm delivery. Most of this confounding should be accounted for by adjusting for time, as both the variation of air pollution and the variation of socioeconomic status are functions of time. Although the risk of confounding from socioeconomic status is limited, the overall performance of the models would likely improve by adjusting for socioeconomic status. Our finding of an early pregnancy effect of O3 is similar, albeit smaller, than the findings of Hansen et al. [13]. Liu et al. found an OR of 1.08 for a 10 ppb increase in first month O3 in their unadjusted model, but this effect was reversed, OR = 0.98 per 10 ppb increase, when adjusting for possible confounders; maternal age, parity, infant sex, birth weight and season of birth [4]. The first finding is similar to the first trimester crude effect in this study, but the results diverge in the adjusted models. The difference in results may be due to difference in how seasons were defined, Liu et al. adjusted for season of birth (which may be modified by preterm birth) while we adjusted for season of conception. No associations between early gestation exposure to O3 were found in the two studies from the southern California [3,5]. The inconsistent findings could be caused by differences in susceptibility (e.g., different anti-oxidant status in different populations), differences in exposure patterns (e.g., high