The teen birthrate has plummeted rapidly since the mid-2000s — and a new study suggests better use of birth control is entirely responsible.

There are 42 percent fewer teen births now than just seven years ago. In 2007, 4.2 percent of teenage girls in the United States gave birth. In 2014, the rate was 2.4 percent.

This is 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 between 15 and 19 said they’d ever had sex in 2007, compared with 45 percent in 2012.

What changed was how teenage girls used contraceptives. The percentage 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 suggests the steep declines in teen pregnancy might not just continue — they might reach new historic lows.

Why over-the-counter birth control is so necessary