Abstracts: ISEE 21st Annual Conference, Dublin, Ireland, August 25–29, 2009: Symposium Abstracts: Symposia Presentations
Many epidemiological studies assign exposure to an individual's residence at a single time point, such as birth or death. This approach makes no allowance for migration and may result in exposure error, reduced study power and biased risk estimates. Pregnancy outcomes, with short exposure windows and lag periods, are less susceptible to this bias; however pregnant women are a highly mobile group. Our objective was to explore the determinants and influence of mobility on characteristics of maternal environment.
We assessed mobility between booking appointment (∼13 weeks gestation) and delivery in northern England using data from the Northern Congenital Abnormality Survey (NorCAS) (1985–2003). Cases with a gestational age at delivery of ≥24 weeks and with known addresses at both booking and delivery were eligible.
Out of 7,919 eligible cases, 705 (8.9% (95% CI 8.3–9.5)) moved between booking and delivery. Movers were younger (25.4 versus 27.3 years, P < 0.01) and from more deprived areas (index of multiple deprivation (IMD) score 38.3 versus 33.7, P < 0.01) than non-movers. In movers, aspects of the maternal environment did change following the move, including IMD score (37.4 at delivery versus 38.8 at booking, P < 0.05) and Townsend deprivation score (3.7 at delivery versus 4.0 at booking, P < 0.05).
Mobility in the north of England (9%) is considerably lower than that reported in North America (20–30%) and the only other study from the UK (23%). Consistent with other studies, mobility was related to maternal age and socio-economic status, and most moves were made locally. While these data support anecdotal evidence that this population is comparatively stable, the mobility we have observed can significantly influence ‘exposure' to various aspects of the maternal environment. Migration should therefore be considered a potential source of bias in studies reliant on postcode at delivery to assign prenatal exposures.