Unintended pregnancies fell 18 percent between 2008 and 2011 — the steepest decline in decades — and it's largely because women are choosing better, more effective birth control.

Since the 1980s, the nonprofit Guttmacher Institute has tracked how many women unintentionally became pregnant. And up until 2008, the number hovered between 49 and 59 unintended pregnancies per 1,000 women ages 18 to 44.

The 2011 data shows a marked change. There were 45 unintended pregnancies per 1,000 women, which is the lowest number the group has ever recorded.

"You see it across all demographics; it's just a pretty sharp drop," says Lawrence Finer, a co-author of the study and the director of domestic research at the Guttmacher Institute.

The story, Finer says, is clear: Women are picking better, more effective birth control. Since 2007, researchers have seen a sharp rise in long-acting reversible contraceptives (LARCs), such as intrauterine devices and implants. These forms of birth control last for years once inserted and prevent pregnancy for more than 99 percent of users. That helps explain why they're a big part of the story behind America's plummeting unintended pregnancy rate.

IUD use has nearly tripled since 2007. That's a huge part of the story.

LARCs like IUDs and implants are amazingly, fantastically good at preventing pregnancy — better than any other available birth control. The fact that usage of these contraceptives has nearly tripled since 2007 is a huge part of why unintended pregnancies are dropping.

Birth control pills and patches, which have to be taken regularly, are susceptible to human error. LARCs are different. Once inserted, they last for at least three years without any work on the patient's behalf.

The birth control pill, one of the most common forms of contraception, has a 6 percent failure rate. So out of 1,000 women taking birth control pills, 60 will become pregnant in a typical year. Among women who use an IUD, that number will be between two and eight (depending on the type of IUD they use). For implant users, 0.5 of every 1,000 users will get pregnant in a given year.

LARCs still aren't the most popular type of birth control in the United States. But usage has increased significantly, from 3.7 percent of birth control users choosing IUDs or implants in 2007 to 11.6 percent in 2012.

That change coincides pretty well with the decline in unintended births, which is why researchers believe these more effective contraceptives are largely to thank.

The big mystery: Why did birth control patterns change when they did?

What drove the rise of LARCs is still a bit of a mystery. It's true that Obamacare expanded access to these types of birth control, by expanding insurance and eliminating copayments for all contraceptives. But the health care law didn't pass until 2010, and its insurance expansion didn't start until 2014. LARC usage began to rise well before then.

Professional medical groups like the American College of Obstetricians and Gynecologists have increasingly endorsed LARCs as the best birth control in official policy statements. ACOG, for example, put out a practice bulletin that recommended LARCs as the "first-line" contraceptive choice for adolescents in 2012. But again, that change and others only happened after LARCs were on the rise.

What drove the rise of LARCs is still a bit of a mystery

Finer says it's possible one important factor might be the introduction of new types of LARCs in recent years, which were more appealing to women. In 2000, Bayer introduced the IUD Mirena. Unlike prior IUDs, which tended to make periods heavier, Mirena actually decreased menstrual bleeding. That could have made IUDs more attractive to patients.

Doctors may have, at the same time, become more informed as they fielded more questions from their patients about LARCs.

"There is a growing understanding that the IUD and other long-acting methods can be used by a wide variety of women, including young women," Finer says. "Those were groups [who] in the past ... weren't really considered the right patients."

Why unintended pregnancies will probably keep dropping

One of the biggest obstacles to LARC use, historically, has been price. Planned Parenthood has estimated that IUDs can cost between $500 and $900 out of pocket. Insurance plans tended to charge patients more for IUDs than for birth control pills, just because the devices have such high upfront costs.

Obamacare is changing that in two ways. The law's insurance expansion means millions more Americans now have coverage, and that will help pay their medical bills. Additionally, Obamacare mandates that insurers cover all contraceptives at no cost to patients. This means that insurers can't charge patients more for an IUD because the device costs more than birth control pills.

There's already evidence that this regulation has lowered financial barriers to LARCs. A 2015 study in the journal Health Affairs found that Obamacare's birth control mandate reduced out-of-pocket spending on IUDs by $248 per patient.

Finer expects something of a "snowball effect" to happen with LARCs: As more women use them, and talk to their friends about their experience, others decide to change contraceptives — and don't face an economic barrier in doing so.

"People now know others who have gotten IUDs and like them," Finer says. "And when they go to the doctor, they're going to have access to a range of options."