The Pew Research Center released a report that examines the changes in demographic characteristics of women who gave birth in 1990 versus women who gave birth in 2008. Authors Gretchen Livingston and D'Vera Cohn examine these trends including ages, marital statuses, and educational attainment of mothers using National Center for Health Statistics and Census Bureau data.

Article: http://pewresearch.org/pubs/1586/changing-demographic-characteristics-american-mothers
Full Report (pdf): http://pewsocialtrends.org/assets/pdf/754-new-demography-of-motherhood.pdf
Pew Research Center: http://pewresearch.org/