correlated, we also adjusted models for trimes- ter-specific exposures (Table 5). Associations found for BTEX and NO2 exposures in the second trimester were more pronounced in the whole cohort and in the two subsets, but only statistically significant among women who spent < 2 hr/day in nonresidential out- door environments. Variance inflation factor values ranged from 1.76 to 1.96 for NO2 and from 2.53 to 2.89 for BTEX, indicating acceptable levels of collinearity in the multi- trimester models. Discussion We found an effect of exposure to BTEX, and to a lesser extent NO2, during the second trimester of pregnancy on birth weight among a subset of women who spent < 2 hr/day in outdoor environments during pregnancy, after controlling for exposure to the same pollutant during the other two trimesters. Exposure to BTEX during the whole pregnancy period was also significantly associated with birth weight for the same subset. The magnitude of the association was higher for BTEX in all the exposure periods. Overall, exposure during the second trimester appeared to be the most harmful, and the association became larger after adjusting for trimester-specific exposures. Identifying critical exposure windows is a research need but a difficult task because of differences in mixture of pollutants across space and time, as well as possible different effects of specific pollutants during specific exposure periods (Slama et al. 2008a). In addition, there is currently a lack of toxi- cologic information to help guide selection of relevant exposure periods for most fetal growth end points (Ritz and Wilhelm 2008). Activitya Indoor a. Home b. Work c. Other people’s houses d. Other indoor environments Outdoor e. Home f. Work g. Other outdoor environments Walkingb Means of transportationc Car Bus Metro/train Total in nonresidential outdoor environmentsd (f + g) Total at home (a + e) Total in indoor environments (a + b + c + d) Traffic-related air pollution and birth weight To our knowledge, this is the first study assessing the relationship between prenatal exposure to ambient BTEX and birth weight, so we cannot compare our results with those of other studies. Regarding NO2, the evidence of a susceptible window of exposure is unclear. Some studies found an adverse effect of NO2 on birth weight in the second trimester of Table 5. Change (coefficient) in birth weight (g) for an IQR increase (µg/m3) in exposure to NO2 and BTEX at the entire pregnancy period and each trimester in 570 newborns from INMA-Sabadell. Birth weight [g (95% confidence interval)] All women (n = 570) Crude NO2 9 Months –6.1 (–43.3 to 31.2) First trimester –4.5 (–45.2 to 36.2) Second trimester –18.1 (–57.5 to 21.4) Third trimester 4.8 (–36.2 to 45.7) BTEX 9 Months –9.6 (–62.8 to 43.6) First trimester –10.9 (–66.1 to 44.3) Second trimester –26.4 (–84.5 to 31.7) Third trimester 6.5 (–47.7 to 60.8) Adjusted (each trimester separately)a NO2 9 Months 8.8 (–23.8 to 41.5) First trimester 3.3 (–33.2 to 39.7) Second trimester 3.7 (–31.1 to 38.4) Third trimester 16.8 (–18.8 to 52.4) BTEX 9 Months –7.6 (–54.9 to 39.8) First trimester –12.0 (–62.0 to 38.0) Second trimester –13.3 (–65.1 to 38.4) Third trimester 2.5 (–45.3 to 50.4) Adjusted (all trimesters together)b NO2 First trimester –7.6 (–56.8 to 41.5) Second trimester –5.8 (–52.7 to 41.0) Third trimester 24.6 (–22.1 to 71.3) BTEX First trimester –19.2 (–101.0 to 62.6) Second trimester –21.7 (–104.6 to 61.3) Third trimester 30.0 (–44.8 to 104.9) 15.1 ± 3.3 4.4 ± 3.8 1.0 ± 1.4 1.0 ± 0.8 0.1 ± 0.4 0.2 ± 0.8 1.4 ± 1.1 0.9 ± 0.9 0.8 ± 0.8 0.6 ± 0.8 0.2 ± 0.9 0.1 ± 0.3 1.6 ± 1.3 15.1 ± 3.4 21.5 ± 1.6 11.2 0.0 0.0 0.0 0.0 0.0 0.3 0.3 0.0 0.0 0.0 0.0 0.3 11.5 20.0 14.5 5.0 0.5 1.0 0.0 0.0 1.0 0.8 0.5 0.5 0.0 0.0 1.0 14.5 22.0 19.6 9.0 3.0 2.0 0.0 0.0 3.0 0.2 2.0 1.5 0.5 0.0 3.0 20.0 23.0 Women who spent ≥ 15 hr/day at home (n = 276) –21.4 (–75.9 to 33.2) –6.1 (–67.4 to 55.2) –45.2 (–103.4 to 12.9) –7.4 (–67.4 to 52.6) –37.2 (–114.5 to 40.1) –27.3 (–110.3 to 55.7) –68.0 (–152.8 to 16.8) –15.8 (–94.7 to 63.0) 8.6 (–37.9 to 55.2) 9.8 (–43.2 to 62.9) –1.5 (–50.9 to 47.9) 15.8 (–35.1 to 66.6) –13.0 (–79.3 to 53.3) –16.3 (–87.8 to 55.2) –22.5 (–95.1 to 50.0) –1.4 (–68.8 to 66.0) 12.7 (–63.0 to 88.4) –22.8 (–90.7 to 45.0) 22.1 (–46.1 to 90.2) –13.4 (–139.9 to 113.0) –42.3 (–161.4 to 77.0) 35.8 (–74.8 to 146.4) 16.0 ± 3.1 0.2 ± 1.2 2.3 ± 1.9 1.6 ± 1.3 0.2 ± 1.0 0.0 ± 0.5 2.7 ± 1.8 1.3 ± 1.1 1.0 ± 0.7 0.9 ± 0.7 0.0 ± 0.1 0.0 ± 0.1 2.7 ± 1.8 16.2 ± 3.0 20.1 ± 2.2 12.0 0.0 0.0 0.0 0.0 0.0 1.0 0.5 0.0 0.0 0.0 0.0 1.0 12.5 17.0 Table 4. Hours/day in specific activities/locations during pregnancy (reported in the 32nd week of pregnancy). aAdjusted for child’s sex, gestational age, season of conception, parity, maternal educational level, maternal smoking during pregnancy, maternal height and prepregnancy weight, and paternal height. bAdjusted for above variables and exposures to the same pollutant during the other two trimesters. *p < 0.05. Weekdays Weekends Mean ± SD 10th Percentile Median 90th Percentile Mean ± SD 10th Percentile Median 90th Percentile See “Materials and Methods” for time–activity questions (a–h). aDifferences between weekdays and weekends are statistically significant for all the activities (Wilcoxon signed ranks test, p < 0.05). bWomen reported specifically the amount of time spent walking as part of the time spent in other outdoor environments. cMean, 10th percentile, median, and 90th percentile values for bicycle and motorcycle categories were 0. dThis activity refers to time spent in outdoor environments other than at the home address. Environmental Health Perspectives • volume 117 | number 8 | August 2009 Women who spent < 2 hr/day in nonresidential outdoor environments (n = 259) –14.4 (–68.5 to 39.6) –1.2 (–61.4 to 59.1) –46.2 (–103.7 to 11.2) 6.7 (–52.2 to 65.6) –61.9 (–140.0 to 17.6) –50.0 (–132.0 to 32.1) –98.5 (–184.0 to –13.0)* –29.7 (–110.7 to 51.2) –18.6 (–66.3 to 29.1) 4.2 (–49.3 to 57.6) –42.3 (–92.1 to 7.4) –9.0 (–60.5 to 42.6) –76.6 (–146.3 to –7.0)* –52.5 (–125.8 to 20.8) –101.9 (–176.2 to –27.6)* –59.7 (–130.9 to 11.5) 54.8 (–18.5 to 128.0) –74.7 (–140.4 to –9.0)* 0.9 (–64.4 to 66.3) 60.8 (–61.8 to 183.5) –137.3 (–252.6 to –22.0)* –16.7 (–123.3 to 89.8) 16.0 0.0 2.0 2.0 0.0 0.0 2.0 1.0 1.0 1.0 0.0 0.0 2.0 16.4 20.5 20.0 0.0 4.0 3.0 0.3 0.0 5.0 2.0 2.0 2.0 0.0 0.0 5.0 20.0 22.5 1325