Maternal residence at time of delivery is sometimes used as a proxy for residence during early pregnancy to estimate environmental exposures. Residential addresses during time of conception through the first trimester were obtained from mothers of 152 infants with congenital cardiac anomalies and 175 controls, and they were compared with the addresses at delivery abstracted from birth certificates. An estimated 24.8% (95% CI = 20.3, 29.9) of women moved between the time of conception and delivery, and the percentage of cases and controls who moved was similar. Use of address at time of delivery may reduce the likelihood of finding an association between a congenital malformation and a maternal environmental exposure.