Many studies of environmental exposures and birth defects use mothers' addresses at delivery as a proxy for the exposure. The validity of these studies is questionable because birth defects generally occur within 8 weeks of conception and the mother's address at delivery may differ from her address early in pregnancy. In order to assess the extent of this bias, we examined the pattern of maternal residential mobility over the span of 3 months prior to conception through delivery, and associated maternal socio-demographic characteristics. We linked Texas subjects from a national case-control study of birth defects with their corresponding records from the Texas Birth Defects Registry and the Texas live birth certificates. Logistic regression analyses were conducted to assess maternal socio-demographic factors related to mobility during pregnancy. Overall, 33% of case and 31% of control mothers changed residence between conception and delivery. The pattern of mobility was similar for both case and control mothers for each pregnancy period. Multivariate analyses indicated that for case mothers, older age (OR=0.39, 95% CI=0.21-0.70), higher household income (OR=0.35, 95% CI=0.18-0.68), Hispanic ethnicity (OR=0.64, 95% CI=0.44-0.92), and higher parity (OR=0.59, 95% CI=0.38-0.94) were indicators of lower mobility during pregnancy. For control mothers, the same pattern of association was present, however, only older age was significantly associated with low rates of mobility. Studies of birth defects using maternal address at delivery as a proxy for maternal environmental exposures during pregnancy may be subject to considerable nondifferential exposure misclassification due to maternal mobility during pregnancy.