"There is lots of good news in the report," said Brady Hamilton, a statistician at the NCHS who led the study.
It's good news because such births are almost always unplanned and the parents are rarely ready to cope with the responsibility of raising a baby. Teenaged moms are also more likely to have babies of a smaller-than-healthy weight or to have stillborn babies.
The study looks at numbers alone and doesn't address changes in teen behavior. But other research suggests that teens are more easily able to get birth control, says Laura Lindberg, a senior researcher at the Guttmacher Institute.
"If anyone tells you they know exactly why this has happened, they are lying," Lindberg said in a telephone interview. "We don't have all the research and behavioral data in place up to 2011."
That said, there are more than a few hints, according to Lindberg.
"We have gone from a social norm where you don't use contraception at first sex to where you do," she said. "Lots of study shows that using contraception at first sex begins a pattern of using it down the road."
Other statistics show that teenage sex is only down slightly, although girls and boys both are having sex later in their teens. "In contrast, there is an increase in contraceptive use, particularly hormonal methods," Lindberg said.
Image: Teenagers with pregnancy test, via Shutterstock