An estimated 37% of all births in the U.S. were the result of unintended pregnancy, remaining steady since 1982, according to a CDC study published in the National Health Statistics Reports. Sixty percent of women who had an unintended pregnancy between 1998 and 2002 failed to use contraception, mostly because they underestimated their risk of getting pregnant.

http://www.webmd.com/baby/news/20120...ies-unintended