## 4 Discussion
### Synthesis with Previous Knowledge  
Our results provide evidence that maternal exposure to carbon monoxide (CO) is associated with an increased risk of LBW in infants. The effect was stronger among babies who were born during the spring-summer season and in the presence of high exposure to O\({}_{3}\). These findings are consistent with the result from a study conducted by [PERSON] et al. [11] in the US. They reported an adjusted OR of 1.31 (95% CI: 1.06-1.62) per 1 ppm increase in average exposure to CO during the 3 rd trimester. A population-based retrospective study conducted in Brazil [41] also found an association between the fourth quartile exposure to CO during the entire pregnancy and the risk of LBW (1.33 95% CI: 0.93-1.90). In a systematic review and meta-analysis conducted by [PERSON] et al. [9], a statistically significant association between maternal exposure to CO and risk of LBW was reported with a summary OR of 1.07 (95% CI: 1.02-1.12) per 1 ppm. In a case-control study conducted in Peru [42], maternal exposure to CO was associated with low birth weight with an adjusted OR of 3.53 (95% CI 0.95-13.23) per \(\geq\)3.82 ppm increase in exposure. Several epidemiological studies have investigated the association between ambient particulate matter and adverse pregnancy outcomes (including LBW); the results of these studies have been summarized in several meta-analyses. In [PERSON] et al. (2015) [43], birth weight measured as a continuous outcome was negatively associated with 10 ug/m\({}^{3}\) increase in PM\({}_{10}\) (-10.31 g, 95% CI: -13.57 to -3.13 g) exposure during the whole pregnancy. [PERSON] et al. [12] compiled data from 14 prospective birth cohorts involving 12 European countries (European Study of Cohorts for Air Pollution Effects (ESCAPE)). An increase of 5 ug/m\({}^{3}\) in prenatal exposure to PM\({}_{2.5}\) and PM\({}_{10}\) was associated with an 18% (1.18; 95% CI: 1.06-1.33) and 16% (1.16; 95% CI: 1.00-1.35) increased risk of LBW, respectively. Even exposures to PM\({}_{2.5}\) concentrations below the minimum recommended level by the European Union were associated with an increased risk of LBW [12]. Our study showed a 9% (1.09; 95% CI: 0.79-1.50) and a 3% (1.03; 95% CI: 0.74-1.43) increase in the risk of LBW in the highest quartile of exposure to PM\({}_{2.5}\) and PM\({}_{10}\) among babies born during the warm season. Our result is consistent with a meta-analysis including 14 original studies conducted by [PERSON] et al. [14]. Theirresult suggested a 9% increase in the risk (1.09; 95% CI: 0.90-1.32) related to a 10 \(\upmu\)g/m\({}^{3}\) increase in PM\({}_{2.5}\) exposure and 2% increase in the risk (1.02; 95% CI: 0.99-1.05) related to a 10 \(\upmu\)g/m\({}^{3}\) increase in PM\({}_{10}\). We found no evidence of any effect related to exposure to NO\({}_{2}\). Our analyses showed an RR of 1.00 for LBW (95% CI: 0.78-1.28) in relation to NO\({}_{2}\) exposure after adjusting for potential confounders during the whole pregnancy. The Norwegian Mother and Child Cohort Study (MoBa) conducted by the Norwegian Institute of Public Health [44] reported a significant negative association between NO\({}_{2}\) and LBW. This result is consistent with our results and with the results from [PERSON] et al. [45]. We provided evidence that high exposure to O\({}_{3}\) increases the risk of LBW\({}_{2500}\) in all babies and the risk of LBW\({}_{3000}\) among the babies who were born in the warm season (March-August) but not among the babies who were born in the cold season. Our results are consistent with the findings by [PERSON] et al. [46]. The Brisbane study with a study period of over 10 years found a similar effect in the single-pollutant model for ozone. They assessed the potential effects of air pollution on birth outcomes applying [PERSON]'s proportional hazards model that provides hazard ratios (HRs) and 95% confidence intervals (95% CI) associated with an interquartile range (IQR) increase in each pollutant.