Exposure by Stage of Gestation

Several studies specifically analyzed whether a stage of gestation for exposure to air pollutants is the most strongly associated with preterm birth. The results have been inconsistent, with some studies reporting significant associations early or late in gestation. Exposure to sulfur dioxide (Bobak, 2000; Mohorovic, 2004), total suspended particulates (Bobak, 2000), and nitrogen dioxide (Maroziene and Grazuleviciene, 2002) in the first trimester, but not the second or the third trimester, of pregnancy was associated with preterm birth. Among women living in a mining district of the Czech Republic who smoked cigarettes during pregnancy, there was an increased prevalence of preterm births for pregnancies conceived in the winter compared with pregnancies conceived in summer. Because winter in this study was characterized by weather inversions that resulted in unusually high concentrations of fine particles dominated by acidic sulfates, genotoxic organic compounds, and toxic trace elements (Dejmek et al., 1996; Sram et al., 1996), a relationship between conception, exposure and preterm birth is suggested..

In contrast, other studies have reported associations between preterm birth and exposures to toxicants later in pregnancy. Specifically, Liu et al. (2003) found that the rates of preterm birth increased with exposure to sulfur dioxide and carbon monoxide during the last but not the first month of pregnancy. In addition, the most significant elevation in the rates of preterm birth among women who lived near roadways in Los Angeles County with heavy traffic occurred for women whose third trimester fell during the fall and winter months, when the level of traffic-related air pollution was the highest (Wilhelm and Ritz, 2003). In one study, preterm birth was associated with exposure to particulate air pollution (PM10) in the last 6 weeks but not the first month of pregnancy (Ritz et al., 2000). Bobak (2000) found no differences in the odds of preterm birth among the trimesters of gestation (adjusted OR range = 1.24 to 1.27 per 50 |ig of increase in the sulfur dioxide concentration/m3). A detailed time-series analysis of exposure to sulfur dioxide and fine particulates (PM10) found suggestive, although not significant, associations of pre-term birth with the levels of PM10 and sulfur dioxide exposures in the last 6 weeks and the last 1 week of gestation (Sagiv et al., 2005).

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