November 29, 2012

U.S. Birth Rate Falls to a Record Low; Decline Is Greatest Among Immigrants

Appendix A: Methodology

Data Sets

Except where noted, data regarding births are obtained from the National Center for Health Statistics (NCHS) using the VitalStats tabulation tool available at: http://www.cdc.gov/nchs/vitalstats.htm.

These vital statistics data reflect information extracted from completed birth certificates for live births, which include the mother’s characteristics at the time of the birth. These tabulations are based upon births occurring in the 50 states and the District of Columbia to people living in the United States in a given calendar year.

Information regarding the composition of the female population of childbearing age (15-44) is derived from the decennial census for 1990, and 2000, and from the American Community Survey for all subsequent years. The American Community Survey data were weighted to reflect intercensal population estimates.

Birth Rates

The primary measure of birth rate used is the general fertility rate (GFR), which is the number of births divided by the number of women of childbearing age in a given year. For the total GFR, the denominator includes a count of all women ages 15 to 44.10

Following the NCHS convention, birth rates for women ages 35 and older include all births to women 35 and older in the numerator, and a count of women ages 35 to 49 in the denominator.

Birth rates calculated by Pew Research for this report may differ from those published by NCHS because NCHS calculates birth rates using Census Bureau population estimates to measure the number of women of childbearing age.

Variables

All variables, including race and ethnicity, are based upon the characteristics of the mother, not of the father or the baby.

Statistics for whites, blacks and Asians are for non-Hispanics only. The Asian population is defined as including both Asians and Pacific Islanders.

Any mother born in one of the 50 states or the District of Columbia is defined as native born in this analysis. All others are defined as foreign born. Reflecting the NCHS VitalStats convention, in this report mothers born in Puerto Rico or other U.S. territories are classified as foreign born.11 These nativity characteristics do not reflect a mother’s citizenship status at her birth, since standard U.S. birth certificates do not collect this information.

A mother is described as married if she is either married or separated. Otherwise she is considered unmarried.

Unless otherwise specified, all references to “teens” consist of ages 10-19. One notable exception occurs in relationship to teen birth rates, which are defined for females 15-19.

  1. In 2010, 99.7% of births were to females ages 15 to 44.
  2. Had the Puerto Rican-born women been classified as “native born,” the overall share of births to native-born women would have increased by 0.61% in 1990 and by 0.42% in 2010.