Preterm Birth Causes, Consequences, and Prevention (2007) / Chapter Skim
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8 Role of Environmental Toxicants in Preterm Birth
8 Role of Environmental Toxicants in Preterm Birth
Pages 229-254
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From page 229... ...
In addition, a number of epidemiological studies have found significant relationships between exposures to air pollution and preterm birth, particularly for sulfur dioxide and particulates, suggesting that exposure to these air pollutants may increase a woman's risk for preterm birth. Studies to date suggest that agricultural chemicals deserve greater attention as potential risk factors for preterm birth.
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From page 230... ...
Analyses and findings of associations between environmental chemical exposures and low birth weight are not discussed. Because differences in mean gestational age at birth may or may not be relevant for preterm birth, depending on whether or not the distribution is affected at the lower gestational ages, studies in which gestational age was used as a continuous variable are discussed only as they contribute to the understanding of preterm birth.
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From page 231... ...
The timing of the assessment may also influence the exposure measure. For example, if the toxicant concentration in the tissue changes over the course of gestation, the toxicant concentrations at the time of birth for preterm births may differ from those for term births simply as a function of gestational age.
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From page 232... ...
. Additionally, studies showed that pregnant women who were socioeconomically dis
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From page 233... ...
. Because socioeconomic disadvantage and African American race are risk factors for preterm birth (see Chapter 4)
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From page 234... ...
but with decreased rates of preterm births for coastal regions (RR 0.77; 95% CI 0.64–0.91)
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From page 235... ...
(Tsai et al., 2003) had an increased risk for delivery of preterm infants, with ORs ranging from 1.03 to 1.41.
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From page 236... ...
, with many estimates being much lower. The relatively small increased risk of preterm birth reflected by the relatively low ORs or RRs may represent a truly marginal increased risk or may be a consequence of the exposure assessment complexities discussed earlier.
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From page 237... ...
Those investigators found that exposure to sulfur dioxide and total suspended particles modified the distribution of gestational age at birth, such that the largest effects were observed with exposures at the youngest gestational ages. Those researchers calculated an attributable risk (the proportion of cases of preterm delivery in the sample attributable to air pollution)
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From page 238... ...
. However, that study had significant limitations, such as the inclusion of only 33 preterm births; the inclusion of only infants who had survived until 2 years of age; and the use of maternal blood samples, the majority of which were taken at an unspecified time in the postpartum period, with the untested assumption that the postpartum and pregnancy serum DDT and DDE concentrations would be interchangeable.
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From page 239... ...
, as well as the pesticides lindane, aldrin, and hexachlorobenzene, in the blood and placentas of women who went into preterm labor (defined as labor during 12 to 32 weeks of gestation in the first study; preterm labor was not defined in the second study) compared with those in the blood and placentas of women who delivered at term (p < 0.001)
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From page 240... ...
. However, among farmers there was an increase of extremely early, midgestation preterm births (between 21 and 24 weeks of gestation)
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From page 241... ...
Moreover, that study found significant associations with grain farming in particular (OR 1.58; 95% CI 1.19–2.09) and also found a pattern of association with the season of the year and a poor grain harvest quality, which led the authors to speculate that mycotoxins produced by molds may induce midpregnancy preterm labor, with the strongest effect on multiplefetus pregnancies.
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From page 242... ...
. Furthermore, there were no significant differences between PCB concentrations in umbilical cord serum from preterm births compared with those from full-term births in a Spanish population exposed to extremely high levels of air pollution (Ribas-Fito et al., 2002)
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From page 243... ...
. In a survey study of cancer and reproductive outcomes in the TCDD-contaminated Russian town of Chapvaevsk, the average rate of preterm labor was reported to be significantly higher in women who lived in Chapvaevsk than in women who lived in other towns in the region (Revich et al., 2001)
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From page 244... ...
Studies that rely on geographical or ecological exposure assessments of community drinking water samples are limited by the inability to account for individual exposure variations. Epidemiological studies of associations between chlorination disinfection by-products and preterm birth or gestational age published before 2002 have been reviewed in detail by Bove et al.
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From page 245... ...
(2004) reported a slight but significantly reduced odds of preterm delivery with maternal exposure to the chlorination disinfection by-products total trihalomethanes, chloroform, and bromodichloromethane.
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From page 246... ...
with each 1 µg of nicotine/gram of hair. In the high-exposure group in the latter study, the odds of preterm delivery were about sixfold higher in comparison with that for the reference group (adjusted OR 6.12; 95% CI 1.31–28.7)
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From page 247... ...
Although noting the weaknesses and the shortcomings of particular studies, Andrews et al. concluded that an adverse effect of lead exposure on preterm delivery was supported because (1)
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From page 248... ...
(1999) also analyzed their data by separate logistic regression analyses for the primiparous and multiparous women using quartile groupings of umbilical cord lead concentrations and adjusting for various known preterm birth risk factors.
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From page 249... ...
Although the evidence is not unanimous, there is sufficient evidence to suggest that maternal exposure to lead results in an increased risk for preterm delivery. The current level of knowledge is inadequate to determine if there may be a risk for paternal exposure to lead for preterm birth.
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From page 250... ...
(2003) reported increased levels of air pollution in neighborhoods consisting predominantly of minority populations and, after adjusting for maternal risk factors that included race-ethnicity, found a small increase in the odds of preterm delivery (OR 1.05; 95% CI 0.99–1.12)
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From page 251... ...
involves increased intracellular calcium due to activation of voltagesensitive calcium channels in the uterine smooth muscle cells (Bae et al., 1999b; Juberg and Loch-Caruso, 1992; Juberg et al, 1995)
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From page 252... ...
Studies to date suggest that exposures to agricultural chemicals deserve greater attention as potential risk factors for preterm birth. In particular, the report by Longnecker et al.
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From page 253... ...
Moreover, if a pollutant's concentration in the tissue or fluid sampled changes as a function of gestational age because of the physiological changes that occur during pregnancy, the concentrations in samples taken at the time of delivery may reflect the gestational age rather than actual differences in levels of exposure.
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From page 254... ...
Furthermore, there is the potential for very large numbers of women to be exposed to particular pollutants, such that an increased risk presented by such exposures could have a significant impact on the population as a whole. Public health therefore plays an important role in regulating such exposures.
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