To understand the evolving landscape for abortion in the United States, you have to consider two seemingly contradictory things.

The first: The overturn of Roe v. Wade has led to unprecedented attacks on abortion, with nearly half of states banning or planning to ban the procedure since June 2022.

Yet the second: The overall incidence of abortion has increased. In fact, according to new data published yesterday by the Guttmacher Institute, a reproductive health research group, there were more abortions in the formal US health care system in 2023 than in any year since 2011. To get even more granular, there were approximately 1 million abortions last year, a 10 percent increase over the number from 2020.

So what gives? How is this possible?

We have some sense of what’s driving the increase

The first underlying factor is that travel out of state for abortion has gone up, offsetting some of the newer restrictions in states with bans. More than 160,000 people crossed state lines to end pregnancies in 2023, per Guttmacher, almost double the number who did so in 2020.

This isn’t to say the burden of state restrictions has been offset.

“Even if people can travel, doing so comes with significant financial and logistical cost,” Isaac Maddow-Zimet, the lead researcher at Guttmacher, told me. “And it’s only been possible because there’s been a lot of support from folks like abortion funds, and we don’t know how sustainable that [funding] will be long-term.”

Another factor explaining the overall increase is that more residents living in states without restrictions are getting abortions.

This seems to have been driven primarily by improved access policies in those states, like expansion of telehealth medicine, more sliding-scale payment options, and wider Medicaid coverage. Some of these efforts began before 2022, though many picked up steam after the Supreme Court’s ruling.

“The broader context is that access anywhere has never been particularly great, and cost has always been a pretty substantial barrier even before Dobbs,” said Maddow-Zimet.

Another factor likely driving the increase is that more people are choosing to end pregnancies through medication abortion — the two-drug combination of mifepristone and misoprostol.

Use of these pills has been trending up over time, but Guttmacher found a particularly big jump over the last three years, where medication abortion accounted for 63 percent of all US abortions in the formal health care system in 2023, up from 53 percent in 2020.

There are a lot of reasons why people might choose to use this effective option, which researchers have found to be safe and with a very low risk of complications.

Some patients lack access to in-person abortion clinics (both in the states that’ve outright banned abortion since 2022 or just in rural and otherwise underserved regions). Other people may simply prefer to end pregnancies in the comfort of their own home or seek out the pills because they can cost less than surgical abortion.

So … what happens next?

Given that so many people are turning to abortion pills, it should come as no surprise that the anti-abortion movement has ramped up its efforts to restrict access to the medication since the overturn of Roe. The US Supreme Court will be hearing a case next week that could lead to even more restrictions on mifepristone, a drug that has been FDA-approved since 2000.

Still, despite these legal threats and the looming possibility of a future Trump administration using the executive branch to further crack down on the pills, thus far advocates for abortion rights have made tremendous gains in expanding access to the medication since Roe’s overturn.

As I’ve previously reported for Vox, it’s never been easier, faster, and cheaper to access abortion medication nationwide, even in red states with total abortion bans.

Some of this is driven by the growth of new shield laws, where blue-state abortion providers can now mail pills to people living in states where abortion is illegal. Some of it is thanks to the growth of e-commerce sites and other international providers, which have collectively and significantly brought down the cost for patients.

Even if the Supreme Court bans mifepristone this summer or places new restrictions on the drug, many of these alternative methods would remain intact, and many abortion providers would switch to providing misoprostol-only regimens.

In other words, the anti-abortion movement is going to have a really hard time stopping people from ending pregnancies they don’t want.

But okay, here’s what we still don’t know when it comes to figuring out why abortion incidence is going up.

The big remaining unknown is whether the increase we’ve seen has been driven in part by people’s shifting childbearing decisions. Do fewer people want to have kids now? Do more people want to delay having kids? Researchers aren’t sure.

Maddow-Zimet tells me this is a particularly difficult thing to measure, and there’s been almost no quality data on changes in people’s intention to have children since Roe’s overturn. (Though we might get some soon with next year’s National Survey of Family Growth.)

An even bigger unknown: How many people wanted to end a pregnancy since Dobbs but weren’t able to? That sort of data is critical to know but has always been extremely difficult to collect, and we still don’t have reliable methods to do so.

Given all these unknowns, not to mention uncertainty with the upcoming presidential election and future funding streams, it’s hard to say how and whether these abortion trends will continue. We’ll be keeping a close watch.

This story appeared originally in Today, Explained, Vox’s flagship daily newsletter. Sign up here for future editions.