ACS fertility data cover a period from the survey month to the same month in the prior year (e.g. June 2002-June 2003). This results in sightly more than 12 months of data and hence a larger number of births than that tabulated on a 12-month calendar year used by NCHS (January to December). This generally results in overall higher fertility rates in the ACS with the exception of rates of Hispanics where no statistical differences were noted.
Differences in data collection procedures between ACS and NCHS may also affect the comparability of fertility estimates by race and Hispanic origin of the mother as ACS respondents self-identify their race and origin.
Source: Census Supplementary Surveys, 2000 - 2001 and American Community Surveys, 2002 - 2003
