Variable Smoking habits Non smoker Moderate smoker Heavy smoker Unknown smoking habits Parity First child 2nd child 3rd child 4th or subsequent child Sex Male Female Season of conception Spring Summer Autumn Winter Year of conception 1988 1989 1990 1991 1992 1993 1994 1995 Total N 87,213 12,251 7,520 8,601 55,295 39,919 14,524 5,852 59,192 56,396 28,505 31,819 28,876 26,388 14,165 14,478 14,719 15,071 15,065 14,640 14,100 13,350 115,588 75.5 10.6 6.5 7.4 47.8 34.5 12.6 5.2 51.2 48.8 24.7 27.5 25.0 22.8 12.3 12.5 12.7 13.0 13.0 12.7 12.2 11.5 4.84 5.36 6.76 8.96 5.33 4.24 6.27 10.34 5.44 5.20 5.43 5.21 5.42 5.25 5.52 4.91 5.08 5.27 5.42 5.46 5.56 5.39 5.32 39.5 (1.8) 39.5 (1.9) 39.3 (2.0) 39.1 (2.3) 39.5 (1.9) 39.5 (1.7) 39.3 (1.9) 38.9 (2.2) 39.5 (1.9) 39.4 (1.8) 39.4 (1.9) 39.5 (1.9) 39.4 (1.9) 39.5 (1.8) 39.5 (1.8) 39.5 (1.8) 39.5 (1.8) 39.4 (1.9) 39.5 (1.9) 39.4 (1.9) 39.4 (1.9) 39.4 (1.9) 57.2 (13.3) 56.9 (13.4) 57.2 (13.4) 56.6 (13.2) 57.0 (13.3) 57.2 (13.3) 57.0 (13.3) 57.1 (13.3) 57.1 (13.3) 57.1 (13.3) 72.8 (6.7) 58.2 (9.3) 42.8 (6.1) 54.4 (9.1) 61.8 (13.1) 50.1 (13.2) 52.4 (12.6) 54.7 (9.8) 57.1 (14.7) 56.1 (9.7) 65.1 (12.9) 60.1 (13.0) 38.3 (5.3) 39.0 (5.5) 39.0 (5.6) 39.3 (5.7) 38.6 (5.4) 38.4 (5.4) 38.5 (5.4) 38.3 (5.3) 38.5 (5.4) 38.5 (5.4) 36.2 (4.5) 36.2 (5.2) 41.4 (5.7) 40.7 (3.5) 43.9 (6.0) 43.6 (5.2) 36.1 (5.3) 37.9 (3.1) 37.8 (2.9) 37.5 (2.8) 36.2 (3.5) 35.0 (5.3) Int. J. Environ. Res. Public Health 2012, 9 278 Table 3. Proportion preterm delivery within levels of covariates in the adjusted analysis. Proportion of total population, % Proportion Preterm Delivery, % Duration of gestation, weeks (sd) Average first Average first trimester O3 trimester NO2 level, µg/m3(sd) level µg/m3(sd) There was a clear association between O3 levels during the first trimester and preterm delivery, both in the unadjusted and adjusted models (Table 4). The association was stronger in the multiple pollutant models than in the crude model. The Odds Ratio, OR, was equal to 1.08 per a 10 µg/m3 increase in O3 in the individual exposure window model, CI = (1.03, 1.13). In the multiple exposure window model the OR was 1.06 per 10 µg/m3 increase in O3, CI = (1.00, 1.13), and the crude OR was 1.03 per 10 µg/m3 increase in O3, CI = (1.01, 1.05). Higher second trimester O3 was positively associated with preterm birth, although the 95% confidence intervals included the in all models. We did not observe any clear association between elevated O3 levels towards the end of gestation and preterm birth. First and second trimester NO2 levels were non-significant with OR < 1. Higher levels of NO2 during the last week at risk of preterm birth was positively associated with preterm birth in all models.