Mary Ziegler is the Stearns Weaver Miller Professor of Law at Florida State University College of Law.

Abortion rights activists await the Supreme Court’s ruling in Whole Woman’s Health v. Hellerstedt on June 27. (Pete Marovich/Getty Images)

Yesterday’s decision in Whole Woman’s Health v. Hellerstedt finally answered a question almost 15 years in the making: How do you know if a law unduly burdens a woman’s right to choose abortion? Before answering this question, Hellerstedt addressed the constitutionality of two parts of Texas’s abortion law. One provision required doctors performing abortions in the state to have admitting privileges at a hospital within 30 miles, and a second regulation mandated that abortion clinics comply with the regulations applied to ambulatory surgical centers. In a 5 to 3 decision, the court struck down both measures. Most obviously, the decision matters because clinics in Texas will stay open, and the constitutionality of similar laws on the books in other states is now far from a sure thing. But Hellerstedt also makes a difference because it sends the clearest message in decades about where abortion jurisprudence is going: toward a battle over facts.

It may seem surprising that the meaning of an undue burden would have remained up in the air for so long. After all, the Supreme Court first set out the test in 1992 in Planned Parenthood v. Casey. As the court held then, a law created an undue burden when it had the purpose or effect of placing a substantial obstacle in the path of a woman choosing abortion.

But Casey and the decisions following it sent conflicting signals about what defined an undue burden. On the one hand, Casey itself stressed the importance of constitutional values on both sides of the abortion question, including women’s interests in liberty, equality and dignity. If the undue-test required respect for these constitutional interests, it seemed that courts would still have to closely analyze the impact of a law, not rubber-stamp the work of legislators. On the other hand, ever since Casey, the court almost never found anything to be unduly burdensome. Most recently, in Gonzales v. Carhart, a majority deferred to Congress’s conclusions about when a law threatened women’s health. Maybe the undue-burden test allowed courts concerned about the impact of a law to take legislators’ word for it.

Hellerstedt puts the court clearly on the side of a more rigorous undue-burden test. Justice Stephen Breyer’s majority opinion clarifies that courts considering an abortion regulation have to weigh the benefits and burdens imposed by a law. That means that legislatures claiming to protect women’s health will need proof that a law actually does so. Aside from Clarence Thomas, the dissenting justices did not object to this conclusion. Instead, they mostly concluded that the court got the facts of the matter wrong. “While there can be no doubt that [the Texas law] caused some clinics to cease operations,” Justice Samuel Alito wrote, “the absence of proof regarding the reasons for particular closures is a problem.” The court divided primarily over how to make sense of the evidence in the case, not on whether the facts should dictate the outcome.

To understand what will happen now requires a quick look at the history of the undue-burden test. In the early 1980s, when Sandra Day O’Connor joined the Supreme Court, members of the antiabortion movement bet that they did not yet have a majority to overrule Roe. Instead, movement members believed that they could make progress by arguing the facts. In City of Akron v. Akron Reproductive Health Center (Akron I), antiabortion briefs made the case that some abortion regulations actually helped women, protecting their mental health and physical welfare. Abortion-rights supporters fired back with proof of their own. This fight brought up key questions that still reverberate today. Who is qualified to weigh in on the safety of abortion? How much evidence must there be before a medical question is definitely settled? How much should women themselves get to say about whether a law benefited them?

In Akron I, the antiabortion movement failed to convince a majority to adopt the undue-burden test, but the questions raised by the case remained open. Later in the 1980s, antiabortion activists fell out of love with the idea that they had embraced in Akron. Some asked the court to cut to the chase and overrule Roe immediately. Other movement members argued that the undue-burden test is really little more than deference to state and federal lawmakers. If legislators said a law did not create an undue burden, that should be good enough. Abortion-rights supporters also viewed the undue-burden test with ambivalence. Some insisted that any adoption of the test would eviscerate abortion rights. Others experimented, collecting proof that certain laws really did eliminate abortion access.

So what does the undue-burden test mean now? Strangely enough, the answer given by Hellerstedt closely resembles the one hoped for today by supporters of abortion rights and longed for by antiabortion leaders in Akron I. That is: Those on both sides will have to pull together extensive, persuasive and often expensive trial evidence about the effect and purpose of an abortion regulation. In Hellerstedt, abortion-rights supporters worked tirelessly to document how the Texas law would have affected women in the state. Now, the court will require this kind of effort any time an abortion case comes to trial, and it will not just be the people challenging abortion regulations who will have to shoulder the burden. In fact, the majority’s reading of the record in Hellerstedt shows just how skeptical the court may be of the proof supporting similar regulations, particularly when lawmakers claim to protect women’s health.

If you think that all of that sounds like a bit of a pain for those contesting the abortion wars, you are probably right. In some ways, Hellerstedt is the kind of decision that might not make anyone happy in the long-term. And in that way, it perfectly captures the spirit of Casey. When the court announced its decision in 1992, lawyers on both sides claimed to have lost. They disliked how the opinion struck a balance between a woman’s right to choose and the state’s interest in fetal life. And no one certainly looked forward to the kind of grueling struggle about the facts Casey seemed to invite. Today’s decision makes clear that we have not seen the last of battles about the medical, scientific and sociological evidence about abortion, both inside and outside of court. And for better or worse, that is just what the authors of Casey had in mind.