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Aim: Differential migration and choice of denominator have been hypothesized to contribute to differences between period prevalence and birth prevalence of cerebral palsy (CP). The purpose of this study was to evaluate the effects of migration and choice of denominator on the prevalence of CP., Method: Data from the Metropolitan Atlanta Developmental Disabilities Surveillance Program and census and birth certificate files were used to calculate various CP prevalence estimates for 2000., Results: The overall CP period prevalence was 3.2 (95% confidence interval [CI] 2.7-3.8) per 1000 8-year-olds and was similar for those born in Atlanta who resided there at age 8 years (3.3; 95% CI 2.7-4.1) and those born outside Atlanta who moved into Atlanta by age 8 years (3.0; 95% CI 2.3-3.9). CP prevalence in these two migration strata was similar by sex and race/ethnicity. CP birth prevalence of 8-year-olds in Atlanta in 2000 was 2.0 (95% CI 1.6-2.5) per 1000 live births in 1992., Interpretation: The authors found no evidence to support the hypothesis that differential in-migration explained higher period than birth prevalence of CP in Atlanta. Comparability of CP prevalence across geographic areas will be enhanced if future studies report both period and birth prevalence., (© The Authors. Developmental Medicine & Child Neurology © 2013 Mac Keith Press.)