Int. J. Environ. Res. Public Health 2012, 9 276 Figure 2. Duration of gestation plotted against birth weight. Table 1. Descriptive statistics of air pollutants and meteorological variables. Time frame Pollutant Mean a SD Median IQR b First trimester O3 57.1 13.3 55.3 20.4 Second trimester O3 56.7 13.7 54.7 20.9 Last week c O3 59.0 17.5 58.4 25.3 Last week d O3 58.2 17.2 57.5 25.1 First trimester NO2 38.5 5.4 38.3 6.0 Second trimester NO2 38.9 5.4 38.6 5.8 Last week c NO2 38.3 8.6 37.6 11.0 Last week d NO2 38.2 8.5 37.7 10.9 First trimester Temperature 7.9 6.3 7.4 12.2 Second trimester Temperature 7.3 6.2 6.3 11.4 Last week c Temperature 7.7 7.2 7.1 12.4 Last week d Temperature 7.4 7.2 6.4 11.9 First trimester Humidity 69.9 9.3 70.9 15.2 Second trimester Humidity 70.5 9.1 71.8 14.7 Last week c Humidity 70.0 11.9 71.1 18.2 Last week d Humidity 69.9 11.7 71.1 18.0 a Units are µg/m3 for the air pollutants, °C for temperature and % for relative humidity. b IQR is equal to the difference between the 75th and 25th percentiles. c Last week of gestation. d Last week at risk of preterm birth.