All states but West Virginia and North Dakota showed significant drops over five years. But the Mountain States of Arizona, Colorado, Idaho, Nevada and Utah saw rates fall by 30 percent or more.

In 22 states, teen Hispanic birth rates plunged at least 40 percent, which was described as “just amazing,” by the report’s lead author, Brady Hamilton of the Centers for Disease Control and Prevention.

What’s driving the declines? No one can say for sure. Experts believe the explanation is complicated and probably varies a bit from state to state. The national figure has been falling since 1991, aside from a brief interruption in 2006 and 2007.

The CDC report released Thursday is based on birth certificates for 2007 through 2011. Last year, the CDC announced the overall improvement in teen births: a record low of 31 births per 1,000 teens ages 15 to 19. That compares with 42 births per 1,000 five years earlier.

“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.