Potential influence of migration bias in birth cohort studies

Objective: To assess the potential bias, due to migration from the community, in a birth cohort study of learning disability in Olmsted County, Minnesota. Material and Methods: The 1976 through 1982 birth cohort consisted of 8,548 children born to mothers who were residents of Rochester, Minnesota, at the time of delivery. The current status of all children was determined for the 1995 to 1996 school year with the resources of Independent School District 535, the Rochester Epidemiology Project, and the Rochester Reading Center. Information from birth certificates was compared between migrants and nonmigrants with use of standard statistical tests. Results: The cumulative probability of migration by age 5 years was 32.2% (95% confidence interval, 31.2 to 33.2%). When migrants were compared with nonmigrants, the distributions of Apgar scores, father's age; gestational age at birth and number of children in the family were virtually identical. In contrast, migrants were more likely to be nonwhite (6% versus 2%), be born to a single parent (11% versus 7%), and have a congenital defect noted at birth (1.5% versus 0.7%). When considered simultaneously in a logistic regression model, the parents of migrants were more highly educated, migrant mothers were younger and had fewer prenatal visits, and migrant children were more likely to be black. Conclusion: This report demonstrates the extreme effort that must be made to assemble a birth cohort in our mobile society. Furthermore, the results suggest that only slight differences exist between migrants and nonmigrants; these are unlikely to compromise the future results of the ongoing learning disability study.