Int. J. Environ. Res. Public Health 2012, 9 279 Table 4. Crude and adjusted ORs for preterm delivery per 10 µg/m3 increase in pollutant concentration. ORs and (95% CI) ORs and (95% CI) from multiple from multiple Time frame Pollutant C(9r5u%de COIR) Epxoplloustuarnet wmionddeolwa.s Epxoplloustuarnet wmionddeolwa.s studied individually studied simultaneously First trimester O3 1.03 (1.01, 1.05) 1.08 (1.03, 1.13) 1.06 (1.00, 1.13) Second trimester O3 1.01 (0.99, 1.03) 1.02 (0.97, 1.07) 1.05 (0.98,1.12) Last week O3 0.99 (0.98, 1.01) 1.01 (0.98, 1.04) 1.02 (0.98, 1.05) First trimester NO2 0.96 (0.92, 1.01) 0.95 (0.88, 1.03) 0.95 (0.87, 1.04) Second trimester NO2 0.95 (0.91, 1.00) 0.96 (0.88, 1.04) 0.93 (0.85, 1.02) Last week NO2 1.05 (1.02, 1.08) 1.07 (1.03, 1.11) 1.06 (1.02, 1.11) a Adjusted for maternal smoking, parity, sex of the child, temperature, relative humidity, seasonal variation and long term trend. Table 5. Crude and adjusted slope estimates for duration of gestation (in weeks) per 10 µg/m3 increase in pollutant concentration. Slope estimates and Slope estimates and (95% CI) from multiple (95% CI) from multiple Time frame Pollutant estiCmrautdee(9s5lo%peCI) Epxoplloustuarnet wmionddeolwa.s Epxoplloustuarnet wmionddeolwa.s studied individually studied simultaneously First trimester O3 −0.02 (−0.03, −0.01) −0.06 (−0.08, −0.04) −0.05 (−0.08, −0.02) Second trimester O3 −0.01 (−0.02, −0.01) −0.03 (−0.05, −0.01) −0.07 (−0.10, −0.04) Last week O3 0.01 (0.00, 0.01) −0.02 (−0.03, −0.01) −0.02 (−0.03, −0.01) First trimester NO2 0.04 (0.02, 0.06) 0.04 (0.00, 0.07) 0.01 (−0.03, 0.05) Second trimester NO2 0.05 (0.03, 0.07) 0.02 (−0.01, 0.06) 0.04 (−0.01, 0.08) Last week NO2 −0.03, (−0.04, −0.02) −0.04 (−0.05, −0.02) −0.04 (−0.06, −0.02) a Adjusted for maternal smoking, parity, sex of the child, temperature, relative humidity, seasonal variation and long term trend. The associations were very similar in the analyses of gestational period. In the fully adjusted model high O3 levels were associated with a shorter duration of gestation, regardless of window of exposure (Table 5). The strongest association, in the multiple pollutant—multiple exposure windows model, was observed for second trimester O3 and duration of gestation. A 10 µg/m3 increase in O3 was associated with a 0.07 weeks reduction of the duration of gestation, CI = (−0.10, −0.04). Higher first and second trimester NO2 levels had a tendency to be positively associated with gestation, but the 95% confidence intervals for the slope estimates excluded the only for the crude models. Exposure to high levels of O3 and NO2 during the last week of gestation was associated with a shorter duration of gestation. The observed association between first trimester O3 and preterm birth was modified by season, the ORs for preterm delivery ranged from 1.07, CI = (0.98, 1.16), during autumn to 1.13, CI = (1.05, 1.21), during spring per 10 µg/m3 increase in O3 during the first trimester (Table 6). The weakest association