Int. J. Environ. Res. Public Health 2012, 9 274 sources (roads). The contrasts in exposure variables may be completely spatial or completely temporal or both, but more spatial than temporal [7,8,10]. The aim of this study was to assess the association between exposure to O3 and duration of gestational and preterm delivery as well as the association with vehicle exhaust, using NO2 as indicator, during different windows of gestation in the Greater Stockholm area. 2. Population and Methods All vaginally delivered singleton births by mothers in the Greater Stockholm area who conceived during 1988–1995 were included in the study (n = 115,588). The number of inhabitants in this area was approximately 1.6 million and the area was approximately 3,500 km2, with approximately 40% of the population residing in the City of Stockholm, an area of approximately 200 km2. The information was extracted from the Swedish Medical Birth Registry and included date of birth, birth weight, length at birth, smoking habits of the mother, parity, gestational age and sex of the child. Birth date and information on gestational age were used to determine date of conception and the period for the first (gestational week 1–13) and second (gestational week 14–26) of each trimester of pregnancy. Since the preterm born have a shorter third trimester, we chose not to calculate exposure for this period of varying duration. We instead calculated exposure during the last week at risk of preterm birth. Gestational age was calculated using information on date of last menstruation and from ultrasound examinations. The air pollution data was provided by the City of Stockholm Environment and Health Administration. We used citywide daily average levels to calculate trimester averages and averages over the last week of pregnancy for NO2 (daily 1-hour maxima) and O3 (daily 8-hour maxima). O3 was measured at one urban background station (Södermalm above roof) and the regional background station (Norr Malma), while NO2 was measured at three different urban background stations (above roof) within a 1.5 km radius (Södermalm, Hornsgatan, Sveavägen). The daily citywide average was calculated from measured concentrations, or when one station was missing, after imputing values from a linear regression against the other stations as for the APHEA-2 Study [19]. Logistic regression was used to examine if the pollutants influenced the risk for preterm birth (<37 weeks gestation). Both crude models and adjusted models were fitted. In order to improve the quality of the models, they were adjusted for: smoking habits (non-smoker, moderate smoker (1–9 cigarettes/day), heavy smoker (10+ cigarettes/day) and unknown smoking habits), parity (0, 1, 2, 3 or more), and sex. We adjusted for time-varying variables; temperature (°C) and relative humidity (%), as they may confound the study of pollution effects. Additionally we adjusted for seasonal trend by fitting a cyclic regression spline on day of year for conception. Finally, we included a long term trend by fitting a cubic regression spline for conception year. We examined the effects of exposure to O3 and NO2 during the first and second trimesters and during the last week at risk of preterm birth. The exposure effects for the different time frames were modelled both individually and simultaneously. We also studied interaction-effects between air pollution and season of conception in the multi-pollutant fully adjusted models. The seasons were defined as; spring (March to May), summer (June to August), autumn (September to November) and winter (December to February). The effect of the pollution exposures on duration of gestation (as a continuous outcome, in weeks) was assessed using multiple linear regression models, where adjustment was made for the same set of