Whether the decennial United States Census accomplishes its goal of accurately counting everyone residing within the country has been a question posed since the first census was conducted.  Awareness of the extent and patterns of error in census counts (commonly referred to as ``undercount’’) not only has important implications for policymakers and Census enumerators, but also for researchers using the data to conduct demographic, social, and economic research.  While the Census Bureau has provided estimates of census undercount for the population as a whole using various methods, this paper focuses on calculating the undercount for the native-born using the technique of Demographic Analysis, which estimates population using information on births, deaths, and migration.  Restricting analysis to the native-born population allows for greater reliance on the more accurate birth and death records and less on those for migration, which are subject to much uncertainty.  I estimate undercount in the 1980, 1990, and 2000 Censuses using individual-level birth and death records from Vital Statistics for those born in the years 1968 and onward.  Undercount is computed by birth cohort, age, and state of birth for two race groups (Black and Non-Black) and the two sexes.  My initial findings show evidence of a larger undercount for the native-born population than what Census finds for the entire population.  I also find a large undercount of children under age 10 in all three censuses, evidence of age clumping at 5-year intervals, and a negative correlation between overall census error and state population.  I explore potential explanations for these results, including the size of native-born emigration, and highlight the implications these patterns have for research using Census data.