The denominator is the total population of all ages, based on the Census Bureau’s vintage 2020 population estimates. The numerator is the population ages 0-2. The numbers are produced by the Census Bureau by adding births and net migration, and subtracting deaths from the results of the 2010 Census. The 2020 Census results were not used for this calculation.
U.S. Census Bureau, Population Division. (2020). Annual state resident population estimates for 6 race groups (5 race alone groups and two or more races) by age, sex, and Hispanic origin: April 1, 2020 to July 1, 2020. https://www.census.gov/programs-surveys/popest/technical-documentation/research/evaluation-estimates/2020-evaluation-estimates/2010s-state-detail.html