The birth rate among teenagers reached another historic low in 2012, government researchers announced Friday, and there is evidence that a switch to more effective means of birth control is a factor.

According to the National Center for Health Statistics at the Centers for Disease Control and Prevention (CDC), the birth rate among young women ages 15 to 19 fell 6 percent last year, to 29.4 births per thousand, the lowest rate in the 73 years the government has been collecting the data. The decline was across all racial and ethnic groups.

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The 2012 number is “a considerable one year drop,” says pediatrician Dr. John Santelli, a professor of population and family health at Columbia University who has no connection to the study. And it follows fairly sizable declines since 2007, when the rate was 41.5 births per thousand young women ages 15 to 19. In fact, except for a small uptick between 2005 and 2007, the teen birth rate has been steadily declining since 1991, when it reached 61.8 births per thousand.

“Our data comes from the birth certificate that parents complete at the hospital and it provides a wealth of information,” says Brady E. Hamilton, a statistician with the National Center for Health Statistics and the lead author of the report. But to figure out why the teen birth rate is falling, “we have to rely on other sources,” Hamilton says, such as surveys that the CDC conducts of high schoolers.

Santelli has studied those and other survey results. “There is not much evidence of a change in abortion use and not much change in sexual activity” since 2003, says Santelli. For example, the percentage of high school kids reporting ever having sexual intercourse was about 54 percent in 1991, according to the CDC survey, declined through 2002, and then held steady at about 47 percent through 2011, the last year of available data.

“What we have seen is greater availability of much more effective birth control methods,” says Santelli. While condom use increased substantially in the 1990s and early 2000s among high schoolers, it actually declined slightly after that, according to the CDC survey. At the same time, medical professionals have increasingly been recommending the IUD, a small, plastic device that is inserted and left inside the uterus to prevent pregnancy, says Santelli. While it does not protect against sexually transmitted diseases, it can be used in combination with a condom, which does offer such protection.

“Young people sometimes use condoms incorrectly, and sometimes they forget to use condoms,” says Santelli. “There is almost zero user error with the IUD. Once it is in place, it works every time.”

Beyond teens, the birth rate for women in their early twenties also declined in 2012, to a new record low of 83.1 births per 1,000 women, while birth rates rose for women in their thirties and early forties.

“People are starting families later and later, and these are historical changes and happening worldwide,” says Santelli. “The last downturn in the economy has accelerated the trend.”