The teen pregnancy rate in the United States is falling dramatically. It just reached a historic low and has dropped by 61% since 1991.
However, nobody really knows why. Theres a lot of hypothesises going around (this being one) but the real reason is probably a combination of many factors. There is probably a positive feedback effect going on as well, lowering teen pregnancy leads to even lower teen pregnancy.
I read an article several months ago that gave (I believe) 9 factors that probably play a role.
Theres also a lot of successful programs going on at a very local level in many cities/towns/states such as this one in Colorado - http://www.cbsnews.com/news/how-colorado-lowered-their-teen-...
Addressing the teen pregnancies caused by birth control failure - The American Academy of Pediatrics just released their recommendations that IUDs should be a first line contraception for teenagers. 10 years ago they were only recommended for parous women (those who have given birth). The importance of this can't be overstated - IUDs are extremely effective as well as reduce the chance of human error to basically zero and require no maintenance by the user.
I strongly caution giving too much credit to any one thing.
Its all very fascinating stuff.