Effect of age-standardization on trends in US teen birth rates, 1981 - 2010

Background: Teen childbearing in the U.S. has generally declined over time; however, several unexplained periods of increase have occurred since 1981. Moreover, trends in teen births vary considerably among states and racial/ethnic groups. A possible explanation for these periods of increase is change in the age structure of 15-19 year olds. To explore this possibility, we examined the impact of age standardization on trends and disparities in teen childbearing. Methods: We calculated age-adjusted (by single year of age) and unadjusted teen birth rates stratified by race/ethnicity from 1981-2010. We then calculated and compared proportional changes in the adjusted and unadjusted rates over notable intervals of increase or decrease: 1986-1991, 1991-1996, 2005-2007, and 2007-2010. Finally, we estimated the percentage of the proportional changes in teen births over time that could be accounted for by changes in the age distribution. Results: From 1986-1996 and 2005-2007, fluctuations in the age structure inflated increases and decreases in teen birth rates, accounting for 8-35% of changes over time and varying considerably by racial/ethnic group. From 2007-2010, variations in the age distribution minimized decreases in rates of teen childbearing by 5-17%. Conclusion: Fluctuations in the population age distribution can account for sizeable proportions of increases or decreases in teen births over time. When examining trends in teen childbearing, researchers should consider using age-standardized rates so that changes are not inflated or masked by variations in the population age structure of teens.