4 reasons why America’s teen birth rate just hit an all-time low

America’s teen birth rate is falling — and fast.

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

That decline is unprecedented, and spans all 50 states. And the states where it is dropping the fastest are diverse: Colorado, Arizona, Georgia, and Florida have all had declines above 40 percent.

Other states are still making steady progress. North Dakota had the slowest drop, at 18.2 percent. Only two other states (West Virginia and Montana) had declines lower than 30 percent.

“It’s exciting and really unprecedented to see these types of declines on a health issue,” says Ginny Ehrlich, chief executive of the Campaign to Prevent Teen and Unplanned Pregnancy. “Last year alone, the decline was 9 percent. Having spent a decade in child obesity prevention, where change happens much slower, this feels monumental.”

Experts like Ehrlich know that the teen birth rate is dropping fast — faster than it has since the federal government began to keep count. They know that this is not about teens terminating more pregnancies; separate data shows the teen abortion rate has steadily declined since the late 1980s.

What they don’t understand is what, exactly, is driving the trend. But they do have lots of ideas.

I’ve spent a lot of time talking to experts who study teen pregnancy. They all told me something pretty similar: We don’t have one clear-cut explanation for why teen births are going down. And if anyone tells you they have the answer, you should be skeptical.

That being said, they did have a lot of compelling ideas about what might be happening, trends that don’t explain the entire decline in teen pregnancies but might tell some smaller part of the story. Here’s a handful of them.

As the economy turned south in the late 2000s, demographers noticed the start of a predictable trend: Birth rates began to drop. That change makes perfect sense and has shown up in prior economic downturns: Couples seem to delay having babies when budgets are tight.

You can see this at the tail end of the chart below. Between 2007 and 2013, the number of babies born each year to mothers of all age groups fell 8.4 percent. That was much slower than the decline in teen births (the lower line, in light blue) — but still a decline, nonetheless.

This theory doesn’t hold up quite as well in 2014, though. Last year, America’s fertility rate rebounded a bit when births to American women increased for the first time since 2007. The teen birth rate, meanwhile, continued its steady march downward.

One of the biggest recent changes in teens’ sexual behavior is the type of contraceptives they choose. Teenagers have increasingly gravitated toward long-acting, reversible contraceptives (LARCs) such as IUDs and implants. The percentage of women using these types of methods grew from 0.4 percent in 2005 to 7.1 percent in 2013 — a 17-fold increase in less than a decade.

Devices like the IUD (inserted into the uterus) or implant (inserted into the arm), once in place, act as continuous birth control for at least five years. This is part of the reason these contraceptives are way better at preventing pregnancy than others. An estimated nine of 100 women taking any type of birth control will get pregnant within a given year. For women using LARCs, it’s fewer than one in 100.

From 2009 to 2013, Colorado ran a massive program where it gave thousands of low-income women access to free IUDs. This helps explain why the state experienced the country’s largest recent decline in teen births: a 52.1 percent drop since 2007.

Every two years, the federal government asks America’s teenagers a series of relatively intimate questions about their sex lives. This is the Youth Risk Behavior Surveillance System, and it shows that high schoolers today have less sex than their parents did when they were teens.

In 1991, when the YRBSS began, 54 percent of high school students told researchers they’d had sex at least once, and 37.5 percent said they were currently sexually active.

By 2013, the most recent year we have data for, those numbers had dropped: 46.8 percent of high school students had had sex, and 34 percent said they were currently sexually active.

To review the data so far: High school students are having less sex than the teenagers of the early 1990s. And when they do have sex, they tend to use better birth control. Those two factors, taken together, almost certainly have something to do with the teen birth rate’s quick decline.

When I spoke with Ehrlich, she pointed out something I hadn’t noticed in the data: The teen birth rate is now way lower for younger teenagers than older teenagers. In 2014, 10.9 of every 1,000 15- and 16-year-old girls gave birth. For teens between 18 and 19, the number was 43.8.

The birth rate for all teenagers is falling. But it’s falling much faster for young teens. Since 2007, births have fallen 47 percent for young teens — and 39 percent for older teens.

To Ehrlich, this suggests there’s something about the environment that young teens are in that is different. She thinks there is something about the high school environment — perhaps better sex education — that is working well for younger women. She points to the Obama administration’s 2010 decision to shift sex education funding to “evidence-based” programs that had shown, in pilots, to change teens’ behavior (whether that means reducing pregnancy or steps toward reducing the odds of pregnancy, like delaying age of first intercourse).