Teen births hit a new low in 2015 — and have fallen by nearly half over the past seven years.

New federal data shows there were 22.3 births for every 1,000 girls ages 15 to 19. That’s a 47 percent decline from the teen birthrate in 2009 — and a 64 percent decline from the teen birthrate in 1991.

The reduction in teen births is arguably one of the biggest public health victories of the 21st century. It’s rare to see such significant changes in a health metric in just a few years. Imagine if obesity rates, for example, tumbled 40 percent in seven years.

That’s what’s happening with teen births.

The decline in teen births is happening across different racial groups, too. The birthrate for black and Hispanic teenagers is still above average but has dropped significantly over the past two decades, and especially since 2009.

Birth control helps explain the huge decline in teen births — and might explain all of it

The decline in teen pregnancy has been an incredibly fast change in a public health trend, and it’s left some researchers puzzled over how it happened so quickly.

But researchers Laura Lindberg, John Santelli, and Sheila Desai say it’s not a mystery at all. Writing in the Journal of Adolescent Health, they find that teenage girls in 2012 were just as likely to be sexually active as girls in 2007. Survey research shows that 43 percent of girls ages 15 to 19 said they’d ever had sex in 2007, compared with 45 percent in 2012.

What changed was how teenage girls used contraceptives. The share of sexually active teens who used at least one type of birth control the last time they had sex rose from 78 percent in 2007 to 86 percent in 2012. More teens gravitated toward better types of birth control — like pills, IUDs, or implants — rather than relying on lower-quality birth control like condoms.

This study only included data through 2012, and, as the researchers note, it’s possible the narrative could be changing.

Newer data that runs through 2015 is starting to show a decline in sexual activity among teenage girls.

"It is unclear whether these new data represent a new trend or are the result of other factors," the study authors write of the new figures.

But if this is the start of a decline in sexual activity among teenage girls, that could help explain why the teen birthrate has fallen even faster in recent years — and why it might continue to plummet in the future.