US women are having abortions at the lowest rate on record since Roe v. Wade, the Supreme Court’s landmark 1973 decision that legalized abortion, according to a new report. In fact, contrary to popular opinion, the abortion rate has been steadily declining for decades.

The new report comes from a massive census of US abortion providers taken every three years by the Guttmacher Institute, a nonprofit research organization that supports legal abortion. It’s surprisingly difficult to get accurate data on abortion in the US (more on that later), but Guttmacher’s census is the most comprehensive available on the subject.

Guttmacher gathered data on abortions performed between 2011 and 2014, and found that abortion rates continued their steady downward trend before reaching historic recorded lows in 2014. The overall rate, 14.6 abortions per 1,000 women, is lower than it was in 1973, when Roe v. Wade was decided and when the abortion rate was 16.3 per 1,000 women.

What’s more, Guttmacher found, the raw recorded number of abortions fell below 1 million for the first time since the mid-1970s. There were 1.06 million recorded abortions in 2011, and by 2014 that number was 926,200.

The abortion ratio — the proportion of abortions to live births — is also down to historic lows. In 1995, the abortion ratio was about 26 abortions for every 100 live births; in 2014, it was 18.8.

The abortion rate and the abortion ratio tell us different things. The abortion rate is a bigger-picture snapshot of how common abortion is among women every year, while the abortion ratio gives us a sense of how many women who get pregnant decide to stay pregnant.

The abortion ratio speaks to a hot-button question when it comes to the politics of abortion: If fewer women are having abortions, is it because they are rejecting abortion from a moral standpoint, because they want an abortion but can’t get one, or because they are having fewer unwanted pregnancies in the first place?

The abortion rate mostly fell because more women used birth control, and used more reliable methods

Abortion rates have been falling for three decades in the developed world, as Vox’s Sarah Kliff has explained. But in developing African, Asian, and Latin American countries, rates have either held steady or increased since the 1990s. That’s because women in developed countries, such as in Europe and North America, have much better access to higher-quality methods of birth control, and live in a culture that treats contraception as less of a taboo.

But abortion rates in the US have been falling even faster than usual since 2008 — 3 or 4 percent per year instead of about 2. And while Guttmacher researchers Rachel Jones and Jenna Jerman caution that more research is needed to fully understand the link between abortion access and abortion rates, better contraception appears to be the main reason.

“Fewer women had abortions in 2011 than in 2008 because fewer women became pregnant when they did not want to,” the researchers write. Over this period, the proportion of pregnancies that were unintended fell from 51 percent to 45 percent.

The best explanation for this sudden drop is that use of long-acting reversible contraceptives (or LARCs) like IUDs and implants increased 130 percent among US women between 2007 and 2009. LARCs are much more reliable and less likely to fail than either birth control pills or condoms (LARCs fail less than 1 percent of the time, compared to 9 percent with typical use of birth control pills or 18 percent with condoms).

That trend likely continued between 2011 and 2014, Jones and Jerman conclude. Low-income family planning clinics supported by the Title X program gave LARCs to 11 percent of their patients in 2014, up from 7 percent in 2011. Low-income and young women have the most unintended pregnancies, and are mostly served by Title X clinics.

The pro-life movement’s explanation for why abortion rates are falling doesn’t fit the data

The abortion rate, like everything else about abortion in America, often gets politicized. Both supporters and opponents of abortion tend to agree that lower abortion rates are a good thing — but for very different reasons.

To abortion rights supporters and most medical providers, lower abortion rates are good because they mean that women are having fewer unintended pregnancies. In this view, abortion isn’t inherently wrong, nor is it possible to get rid of entirely — even the best birth control methods fail sometimes, and health complications or fetal abnormalities mean that not every pregnancy can be carried to term. But it’s best for women if they can control their fertility from the outset with access to reliable, affordable birth control.

Abortion opponents think abortion is wrong because it ends a human life, and thus shouldn’t be legally allowed or morally tolerated. In their view, the best way to reduce abortion is to restrict it legally while also working to change a broader culture that accepts legal abortion.

That’s one reason state governments that are hostile to abortion have passed hundreds of laws restricting abortion since 2011, and more than 1,000 since Roe v. Wade was decided.

"These [laws] have been game-changers, and we see the abortion rate dropping in response," Kristi Hamrick, a spokesperson for the anti-abortion lobbying group Americans United for Life, told NPR.

But while some of these laws may have lowered abortion rates in some states, they can’t possibly explain the overall drop, according to an analysis of the new census by Guttmacher’s Joerg Dreweke.

Twenty-two states enacted a total of 47 new restrictions in 2012, 2013, and 2014 that could have been significant enough to impede abortion access. But those 22 states only accounted for 38 percent of the total abortion rate decline between 2011 and 2014.

One particular category of restrictions known as TRAP laws (targeted regulation of abortion providers) may have reduced abortions in the states they were passed, simply because they made abortion more expensive or harder to get for many women. Of the nine states that implemented new TRAP laws between 2011 and 2014, six had larger-than-average declines in the abortion rate.

But again: 62 percent of the abortion rate decline came from the 29 states and jurisdictions (including the District of Columbia) that didn’t pass restrictive abortion laws during this time period.

The only explanation that really makes sense across the board is better access to birth control.

Hamrick also argued that public sentiment is turning against abortion — but two decades of surveys from the Pew Research Center don’t support that view. Opinions about abortion have stayed relatively stable, and about seven in 10 Americans don’t want to see Roe v. Wade overturned.

This new data has some limitations, but it’s the best out there

The Centers for Disease Control and Prevention (CDC) has been compiling data on national abortion rates since 1969, but that data can be inconsistent. States aren’t legally required to report their abortion statistics to the federal government, and health department data collection methods are inconsistent from state to state.

Guttmacher goes to a lot more trouble than the CDC to get comprehensive data. Researchers mail surveys to the “known universe” of abortion providers, including potential new providers that they work to identify. They follow up with multiple mailings, phone calls, faxes, and emails to the providers that don’t respond.

In the end, Guttmacher researchers got direct abortion data from 58 percent of the facilities they contacted — which accounted for 88 percent of the abortions they counted in the 2011-’14 survey. They estimated the rest based on state health department data and reasonable projections from other sources of data.

So 12 percent of Guttmacher’s abortion count may not be exact. And other surveys Guttmacher has performed suggest that the researchers could be missing about 2,000 OB-GYNs who sometimes perform abortions as part of their private practice.

Still, Guttmacher’s data is about as comprehensive as it gets when it comes to measuring the incidence of abortion in America. And according to that data, fewer US women are getting abortions than ever — and better access to contraception seems to be the main reason.

Watch: Why over-the-counter birth control is so necessary