I'm not sure it's been long enough to tell if these shows are truly causing a decline in teenage pregnancy. I'm also not sure that these shows are really the cause. There are a ton of other things that have changed in the last 5 years besides these television shows. One that I can imagine is simply the access to information and birth control. 20 years ago, teenagers had to go work hard to get information - go to the library, hope parents and teachers provided information, etc. I'm also not sure how abstinence only education vs family planning education has changed over the same time period. Today, 11 year olds have iPhones with a 24/7 connection to the internet. It's certainly easier to Google birth control, etc than it was before. It's also easier to see how close the nearest Planned Parenthood or woman's clinic is.
I'd be interested in seeing how sex Ed has changed in secondary school, if it has changed at all. When I was in high school it was still abstinence based, hopefully that's not the case anymore. I don't think they're a major cause, but I think they help by showing that early pregnancy isn't cool and you (the viewer) probably isn't ready for it.