ReviewAmbient air pollution, birth weight and preterm birth: A systematic review and meta-analysis
Introduction
Preterm birth and low birth weight are well-known to be associated with increased neonatal morbidity and mortality as well as possible increased morbidity in adulthood (Behrman and Butler, 2007). In 2005, the prevalence of low birth weight in the US was 8.2% and that of preterm birth was 12.6%; 67% of low birth weight infants were born preterm, while 43.3% of preterm infants were low birth weight (Martin et al., 2007). Complications of preterm birth include both early effects on the respiratory, gastrointestinal, immunologic and central nervous systems as well as late effects on motor, cognitive, visual, hearing, behavioral, and social-emotional function, and diverse effects on health and growth (Behrman and Butler, 2007). The annual economic burden associated with preterm birth in the United States in 2005 was estimated to be at least $26.2 billion ($51,600 per infant), approximately 65% of which ($16.9 billion or $33,200 per infant) was attributed to medical care (Behrman and Butler, 2007).
Numerous studies have been conducted of the association between ambient air pollutants and pregnancy outcome, including preterm birth and birth weight. Several reviews have been published (Bonzini et al., 2010, Bosetti et al., 2010, Ghosh et al., 2007, Glinianaia et al., 2004, Maisonet et al., 2004, Shah and Balkhair, 2010, Sram et al., 2005, Stillerman et al., 2008), but they have generally been based on a small number of studies, and only one has provided pooled estimates of effect size for particulate pollutants only (Sapkota et al., 2010). Examination in primary studies of varied combinations of pollutants and exposure periods (month, trimester and other periods), and treatment of exposure as both a continuous and categorical variable, have made it difficult to generalize about the true nature of these associations. This is reflected in the conclusions of previous reviews, which have generally characterized effects as heterogeneous and at best small in magnitude. We present a systematic review and meta-analysis of over 60 studies examining associations between ambient air pollution and birth weight and preterm birth. We provide summary estimates of effect by gestational period, quantify heterogeneity, evaluate publication bias, and conduct meta-regression and numerous sensitivity analyses.
Section snippets
Study identification and data extraction
Studies were identified using electronic searches of bibliographic databases with assistance from a science librarian, and review of reference lists of all relevant papers. The following databases were searched: EMBASE, MEDLINE, Scopus, Current Contents, Global Health, Cochrane, TOXLINE and the Canadian Research Index. Search terms were based on the following inclusion criteria: non-occupational non-accidental exposure to outdoor nitrogen dioxide (NO2), sulfur dioxide (SO2), ozone (O3), carbon
Descriptive summary of studies
After review of the title and abstract of 2026 unique records identified in the literature search, 301 were selected by either reviewer for review of the full paper to determine eligibility for inclusion. 61 studies met the inclusion criteria and one additional study was identified through review of the reference lists of relevant papers (Fig. 1). Characteristics of individual studies are summarized in Table 1. Most employed a retrospective cohort design using administrative birth record data,
Discussion
We observed pooled estimates of decrease in birth weight of approximately 10–30 g, and odds ratios of 1.05–1.10 for low birth weight and 1.04–1.06 for preterm birth attributable to representative concentrations of CO, NO2, PM10, and PM2.5. Results were less consistent for O3 and SO2. We found that pooled estimates were generally insensitive to the inclusion of additional results based on term IUGR and SGA, those derived from categorical exposures, and substitution of alternative estimates based
Conclusion
Based on a systematic review and meta-analysis of 62 studies of air pollution and pregnancy outcome, we conclude that while there is a large evidence base, and pooled estimates of effects are generally indicative of associations between CO, NO2, PM and pregnancy outcome, there was considerable variability in pooled estimates of effect by exposure period, and often a high degree of heterogeneity between studies. Meta-regression for birth weight and selected pollutants indicated that
Conflict of interest
None declared.
Acknowledgments
The authors thank Joanne Johnson for assistance with literature searches and Drs. Michelle Bell, Michael Brauer, Lei Chen, Nelson Gouveia, Simone Gray, Craig Hansen, Shiliang Liu, Christian Madsen, Stanley Omaye, David Rich and Beate Ritz for providing additional information on their studies. Funding was provided by Health Canada.
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