What has contributed to the massive drop in teen birth rates?

What has contributed to the massive drop in teen birth rates? Teen pregnancy has decreased much more quickly between 2007 and 2013 than it did 20 years ago, but the decline “has proved difficult to explain.” Theories include expected hypotheses like teens using contraceptives more, to more offbeat concepts like TV series Teen Mom acting as a deterrent. The availability of Plan B, better access to information on the internet, and even a theory that links changes in gasoline to reductions in teen pregnancy are also being discussed. (Vox)