The CDC credits the downturn to changing expanded economic opportunities for teens during the boom times of the 1990s and the increase in condom use by teens as awareness of HIV and AIDS grew.
Stephanie Ventura is a demographer for the National Center for Health Statistics, which produced the study. She explains a few of the intricacies of assembling the data.
TIME.com: These numbers come from tallying up live births and adding in estimates for miscarriages and abortions. How did you get those?
Stephanie Ventura: We get the rates of abortion from two sources. The first is the CDC Division of Reproductive Health, which uses a voluntary reporting system. State health departments provide counts and rates of abortions to them. The second is the Alan Guttmacher Institute, which conducts an abortion provider survey. They know about people who do relatively few abortions, who may not report to the state, so their numbers are higher that the state health departments'.
We adjust the figures from the CDC to the Guttmacher system. The CDC, in their reports, acknowledges that the state health department data they get is about 10 to 12 percent incomplete.
The miscarriage estimates come from the National Survey of Family Growth conducted by NCHS. It's a nationally representative sampling of about 10,000 women ages15 to 44, large enough to estimate statistics for teens and minorities. The survey asks about all aspects of fertility and family status situation, including sexual activity and contraception. One part is pregnancy histories. Women who had a miscarriage in their second or third months report it there. Most states don't require that miscarriages before 20 weeks be reported, so results of all pregnancies that women know about are more complete.
And what is the importance of this type of study?
This is a study that we do every year, an ongoing process. The most recent abortion data that we have is from 1997, but we have teen birth rates through 1999, and the rate is continuing to fall. We're quite confident that if we had information for more recent years the rates are still declining.
In terms of welfare reform and teen pregnancy prevention programs, it's a way of knowing whether the programs are having any effect. It's an important measure, one that we'll be looking at for the foreseeable future. It's good news that the teen pregnancy rates are falling, but they're still high, much higher than in any other developed country and much higher than they should be.