Ambient air pollution exposure, residential mobility and term birth weight in Oslo, Norway,☆☆

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Abstract

Environmental exposure during pregnancy may have lifelong health consequences for the offspring and some studies have association between maternal exposure to air pollution during pregnancy and offspring’s birth weight. However, many of these studies do not take into account small-scale variations in exposure, residential mobility, and work addresses during pregnancy.

We used information from the National Birth Registry of Norway to examine associations between ambient environmental exposure such as air pollution and temperature, and offspring’s birth weight taking advantage of information on migration history and work address in a large population-based cohort. A dispersion model was used to estimate ambient air pollution levels at all residential addresses and work addresses for a total of 25,229 pregnancies between 1999 and 2002 in Oslo, Norway.

Ambient exposure to traffic pollution for the entire pregnancy was associated with a reduction in term birth weight in crude analyzes when comparing children of the highest and lowest exposed mothers. No evidence for an association between exposure to traffic pollution at home and work addresses and term birth weight after adjustment for covariates known to influence birth weight during pregnancy. After stratification, small statistically non-significant reductions were present but only for multiparious mothers. This group also had less residential mobility and less employment during pregnancy.

The overall findings suggest no clear association between term birth weight and traffic pollution exposure during pregnancy. However, mobility patterns could introduce possible confounding when examining small-scale variations in exposure by using addresses. This could be of importance in future studies.

Introduction

Birth outcome, and especially low birth weight, is considered as an important predictor of children’s health (McCormick, 1985; Wilcox, 2001), later life health such as coronary heart disease (Barker, 1995), and other health problems (Ashdown-Lambert, 2005). In recent years, several studies have examined the associations between birth outcomes and exposure to environmental factors such as tobacco smoking, ambient and indoor air pollution (Brauer et al., 2008; Nethery et al., 2008; Bell et al., 2007; Ritz and Yu, 1999; Maisonet et al., 2001; Dejmek et al., 1999; Baxter et al., 2007; Jedrychowski et al., 2006; Rauh et al., 2004). The findings on the association between air pollution exposure during pregnancy and pregnancy outcomes have been summarized in recent reviews (Ritz and Wilhelm, 2008; Lacasaña et al., 2005; Šrám et al., 2005; Maisonet et al., 2004; Glinianaia et al., 2004). The authors conclude that there is supportive evidence of associations between ambient air pollution and birth weight. However, the results are not conclusive, and the causal mechanisms are not yet clear.

Differences in study design and inclusion criteria, and different approaches in assessing exposure make it difficult to compare results between studies (Slama et al., 2008; Parker and Woodruff, 2008). A common approach in these types of epidemiological studies is to define exposure for the entire pregnancy based on the mothers registered address at the time of birth. Studies have shown that in some societies, up to 30% of women move during their pregnancy (Khoury et al., 1988; Fell et al., 2004; Shaw and Malcoe, 1992; Canfield et al., 2006). Using only the residential address registered on delivery could give raise to misclassified exposure estimates due to change of residency during pregnancy, since one can expect increased variability in exposure when using additional addresses for each pregnant woman (Ryan et al., 2008).

The purpose of this study was to use a population-based approach to examine associations between ambient environmental exposure, residential mobility and term birth weight. We hypothesized that reduced birth weight would be associated with higher exposure levels of ambient air pollution at both home and work address during pregnancy. We also wanted to test the hypothesis that differences in residential mobility during pregnancy could express additional unmeasured exposures and thereby affect any possible associations.

Section snippets

Design and study population

The target population consists of all singleton term live births with at least 37 weeks of completed gestation. The included births were registered between 1st January 1999 and 31st December 2002 in Oslo, Norway. We excluded births with maternal address outside Oslo during the pregnancy (n=715), plural deliveries (n=1046), term births with weight <1000 grams, or births with missing information on offspring’s gender or weight (n=119). Pregnancies with missing exposure on ambient air pollution

Results

Table 1 provides descriptive statistics of the study population. A total of 25,229 term live births were included in this study, with 1.2% being LBW and 9.6% being SGA. The mothers were predominantly western ethnic (75.7%) with a mean age of 30.7 years compared to 28.7 years for non-western ethnic mothers. A majority of the mothers were either married or living with a partner (83.3%). LBW was associated with female infants, shorter gestational age, first in birth order, maternal tobacco use,

Discussion

This study used a population-based sample with a detailed residential and pregnancy history for around 80% of all pregnancies in the study area. We used a dispersion model for air pollution exposure to assign residence-based concentrations in air pollution and did not rely on personal reporting of exposure to air pollution. We were able to receive registry-based information on moving date and included several microenvironments in the time weighting of exposure for each individual. By using a

Acknowledgements

The authors wish to acknowledge the services of The Medical Birth Registry of Norway. We also want to thank Statistics Norway for preparing the data.

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    Funding sources: This work was funded as a part of a doctoral thesis by the Norwegian Research Council. The names and personal ID numbers of participants were omitted from the data set to ensure confidentiality. The Norwegian Data Inspectorate approved the study, and the research was conducted in full accordance with the World Medical Association Declaration of Helsinki. Ulrike Gehring was supported by a research fellowship of the Netherlands Organization for Scientific Research (NWO).

    ☆☆

    Approvals: The Norwegian Data Inspectorate reviewed and approved this study.

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