Maternal migration during pregnancy in England and Wales: implications for epidemiological studies of pregnancy outcomes

Oliver Morgan and Ben Armstrong, London School of Hygiene & Tropical Medicine, and Helen Dolk, University of Ulster

[Project number 20073]

Epidemiological studies of pregnancy outcomes, such as low birth weight or congenital anomalies, are often interested in exposures experienced by both the mother and foetus (1 2). In England and Wales, exposures during pregnancy are usually estimated using the mother’s postcode of residence at time of birth. Postcode of residence is routinely recorded on the birth certificate. However, this may not provide an accurate estimation of exposure if the mother moves home during her pregnancy. The consequence of this maternal migration is to introduce exposure misclassification bias into epidemiological studies (3 4).

There are currently no published estimates of maternal migration during pregnancy in England and Wales. Two published studies from the United States estimated that between 20-25% of women move home during pregnancy (5 6). An unpublished study in England and Wales estimates this to be about 23% (7). Migration is likely to vary in different parts of the country and also depending on factors such as socio-economic status. A better understanding of maternal migration and the factors that influence it are needed to interpret future epidemiological studies of pregnancy outcomes in England and Wales.

1. Elliott P, Briggs D, Morris S, de Hoogh C, Hurt C, Kold Jensen T. Risk of adverse birth outcomes in populations living near landfill sites. BMJ 2001;323:363-368.
2. Dolk H, Vrijheid M, Armstrong B, Abramsky L, Bianchi F, Garne E. Risk of congenital anomalies near hazardous-waste landfill sites in Europe: the EUROHAZCON study. Lancet 1998;352:423-427.
3. Armstrong B. Effect of measurement error on epidemiological studies of environmental and occupational exposures. Occupational and Environmental Medicine 1998;55:651-656.
4. Schulman J, Selvin S, Shaw G, Malcoe L. Exposure misclassification due to residential mobility during pregnancy in epidemiologic investigations of congenital malformations. Archives of Environmental Health 1993;48(2):114-119.
5. Khoury M, Stewart W, Weinstein A, Panny S, Lindsay P, Eisenberg M. Residential mobility during pregnancy: implications for environmental teratogenisis. Journal of Clinical Epidemiology 1988;41(1):15-20.
6. Shaw G, Malcoe L. Residential mobility during pregnancy for mothers of infants with or without congenital cardiac anomalies. Archives of Environmental Health 1992;47(3):236-238.
7. Dolk H. The influence of migration in small area studies of environment and health - migration during pregnancy. UPDATE - news from the LS User Group 1997;17:6-8.

The aim of this study is to describe patterns of maternal migration during pregnancy in England and Wales and consider the implications for epidemiological studies of pregnancy outcomes. The following analysis of the LS will contribute to this study:

  • The proportion of women moving during the first, second and third trimesters of pregnancy will be identified by comparing area of residence at birth and at the census.
  • Migration will be considered at regional, local authority and ward levels. Migration distance between and within areas will be calculated using postcodes, for 1991 and 2001 data.
  • Demographic (age, ethnicity) and socio-economic characteristics will be identified from the census data. A regression model will be used to assess the influence of these factors.
  • Migration patterns of pregnant women will be compared to migration patterns of women who were not pregnant.

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