Exclusion Criteria: Records for which birth weight are unknown are excluded (<1% of all records for each year).
The 2010 data are preliminary; the data for 2000-2009 are final. Data for 2010 are from Hamilton BE, Martin JA, and Ventura SJ. “Births: Preliminary Data for 2010.” National Vital Statistics Reports; vol 60, no 2. Hyattsville, MD: National Center for Health Statistics. 2011. These data are available from: www.cdc.gov/nchs/data/nvsr/nvsr60/nvsr60_02.pdf and www.cdc.gov/nchs/data/nvsr/nvsr60/nvsr60_02_tables.pdf
This measure is calculated by the Centers for Disease Control and Prevention, National Center for Health Statistics from the birth data file.