Int. J. Environ. Res. Public Health 2012, 9 277 Table 2. Correlation between O3, NO2 and meteorological averages. First Second Last week a trimester trimester O3 O3 O3 Second trimester O3 0.13 Last week a O3 −0.45 −0.34 Last week b O3 −0.32 0.71 −0.50 First trimester NO2 −0.43 0.16 −0.03 Second trimester NO2 −0.33 −0.39 0.30 Last week a NO2 0.11 −0.26 0.29 Last week b NO2 0.00 −0.18 0.29 tFeimrsptetrriamtuerseter 0.62 −0.61 0.06 tSeemcopnerdattruirmeester 0.69 0.56 −0.73 Lteamstpwereaetukrae −0.79 −0.08 0.54 Lteamstpwereaetukrbe −0.78 0.17 0.27 hFuirmstidtriitmyester relative −0.82 0.24 0.26 rSeelcaotinvde thruimmeisdtietyr −0.30 −0.79 0.69 Lhuamstiwdietyek a relative 0.55 0.41 −0.71 Lhuamstiwdietyek b relative 0.65 0.23 −0.49 a Last week of gestation. b Last week at risk of preterm birth. Last week b O3 First Second trimester trimester NO2 NO2 Last week a NO2 Last week b NO2 0.11 0.30 −0.34 −0.26 −0.18 −0.77 0.61 0.54 0.46 0.61 −0.62 −0.70 0.25 −0.09 −0.07 −0.53 −0.10 0.44 0.49 0.42 −0.11 −0.30 −0.41 0.03 0.03 0.09 −0.50 0.36 0.27 0.19 0.40 −0.36 −0.30 0.48 0.20 0.37 −0.40 −0.32 −0.20 −0.28 0.22 0.28 0.28 0.33 −0.39 −0.40 −0.25 −0.19 0.20 0.23 The proportion of preterm births was higher among smoking mothers (Table 3). The proportion of preterm births was higher in the subset of women who did not have any information on smoking than in the other subsets. The proportion of preterm births was higher among primiparous women than for women giving birth for the second time. Among women giving birth to their third or subsequent child the proportion of preterm births was higher than among primiparous women or women giving birth to their second child. There was only a minor difference between boys and girls in the proportion of preterm birth. There was no clear seasonality in how preterm births were distributed over the year, there was however a slightly elevated proportion of preterm births among the mothers who conceived during Spring or Autumn (Table 3). The proportion of preterm births was somewhat lower during 1989 and 1990, the two years with the lowest first trimester O3 averages, compared to the other years, and the proportion of preterm births was slightly elevated in 1994, when first trimester O3 levels were the highest in this study.