Guttmacher doesn't seem all that perplexed:
U.S. TEEN PREGNANCY, BIRTH AND ABORTION RATES REACH HISTORIC LOWS wrote:
“The decline in the teen pregnancy rate is great news,†says lead author Kathryn Kost. “Other reports had already demonstrated sustained declines in births among teens in the past few years; but now we know that this is due to the fact that fewer teens are becoming pregnant in the first place. It appears that efforts to ensure teens can access the information and contraceptive services they need to prevent unwanted pregnancies are paying off.â€
Quote:
During this same time period, increasing proportions of women aged 18–19 reported having ever had sex, yet fewer of them were getting pregnant. Changes in contraceptive use are likely driving this trend. There is evidence that contraceptive practices are improving among older teens: A recent study found that the proportion of 18–19-year-old women reporting use of long-acting reversible contraceptive methods tripled between 2007 and 2009, and promotion and acceptance of these methods for teens and young adult women has increased. Contraceptive use may also be associated with the large decline in the pregnancy rate among sexually experienced teens.
[...]
Teen pregnancy rates in all 50 states have steadily declined over the past 20 years, and the dramatic decline from 2008 to 2010 in national rates of pregnancies, births and abortions reflects a decline in each of the 50 states. Yet even with long-term and ongoing declines, substantial disparities between states remain in these rates and in the patterns of pregnancy outcomes. A number of factors likely contribute to differences between states, including differences in the demographic characteristics of their populations, the availability of comprehensive sex education, knowledge about and availability of contraceptive services, and cultural attitudes toward sexual behavior and childbearing.
Granted we can't
prove the cause since we can't run experiments with control groups where one group of teens gets condoms and the other doesn't but they don't seem to be scratching their heads and saying "Ain't that a corker?" either.