Background: Pregnant women are a highly mobile group, which may introduce exposure misclassification if exposure during pregnancy is assigned to place of residence at birth. The spatial and temporal variability of the exposure variable, and scale at which it is measured, are likely to influence the extent of error introduced. Aims: We investigated the impact of maternal residential mobility on exposure to a selection of environmental and socio-economic variables that varied in space and/or time. Methods: For spatially varying exposures (deprivation, urban fabric, roads and air pollution (PM10 and NO2)) we compared exposure assigned to maternal residential postcode at delivery versus conception; for exposures with temporal variability (air pollution and temperature) we also compared exposure assigned to date at delivery versus conception. We used data on residential history for deliveries recorded by the Northern Congenital Abnormality Survey, 2000-2008, (n=7432). Results: In this population, 24.4% of women moved during pregnancy. For spatially varying exposures, there was good agreement between exposures assigned to address at delivery versus conception. Nonetheless, depending on the exposure, between 1 and 9% of women overall were assigned an exposure at delivery ＞1SD different to that at conception. For time varying exposures, 18-63% of women were assigned an exposure at the date of delivery ＞1SD different to that at date of conception. Conclusions: Antenatal environmental and area-level exposures assigned to place of residence at birth may be affected by mobility during pregnancy and therefore potentially attenuate or bias risk estimates. However, the extent of the misclassification differs depending on the exposure being considered.
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