In the 1973 landmark Roe v. Wade case, the Supreme Court gave constitutional protection to a woman’s right to an abortion. But in 1992, the court in Planned Parenthood v. Casey amended that right somewhat. States could restrict abortions to protect women’s health as long as those limits did not create an “undue burden.”

Now, 43 years after the landmark decision in Roe, the court must decide exactly what that phrase means.

The undue burden standard sits at the center of the Whole Woman’s Health v. Hellerstedt case heard by the Supreme Court earlier this month. Abortion rights groups claim a 2013 Texas law intentionally all but bans abortion in the state by creating regulations that they say most clinics cannot meet, forcing them to close. Supporters, mostly anti-abortion groups, argue the aim is to raise the level of care so the procedures are safer. The two provisions of the law being challenged are admitting privilege requirements, which mean doctors must have rights of admission at nearby hospitals, and a requirement that clinics must meet the higher standards of ambulatory surgical centers.

As defined by 1992’s Planned Parenthood v. Casey, abortion restrictions become an undue burden on women when they pose “significant obstacles” not supported by a legitimate interest in protecting women’s health.

However, there is almost no precedent from previous Supreme Court rulings to guide states and lower courts when setting or judging abortion regulations — meaning states are open to interpret the standard in a way they see fit.

“The core problem is that undue burden is inherently vague,” said University of San Francisco law professor Maya Manian. “Your opinion on whether you agree with the Texas reading of undue burden probably depends on your view of abortion.”

Federal courts have split on the definition of undue burden, throwing it to the Supreme Court to provide the final judgment. The 5 th U.S. Circuit Court of Appeals ruling in favor of Texas in this case conflicts with rulings by the 7 th and 9 th circuit appeals courts.

The 7 th U.S. Circuit Court of Appeals is led by Reagan-appointed Judge Richard Posner, whom Manian described as one of the most respected jurists in the country. Posner handed down a ruling in 2014 against a Wisconsin law requiring doctors performing abortions to be able to admit patients to nearby hospitals, citing a lack of evidence that the law had a rational basis in protecting women’s health.

The case also is being considered at a time when the court lacks a tie-breaking ninth justice, following the death of Justice Antonin Scalia, who was generally a reliable vote for the conservatives on the court.

Texas Solicitor General Scott Keller said during the March 2 Supreme Court oral arguments that his state doesn’t have to prove its regulations have improved women’s health. Instead, he argued Texas need only prove that passing a law to better protect women’s health was a constitutionally valid use of the state legislature’s lawmaking power.

Manian said the 2007 Gonzales v. Carhart ruling, which upheld a federal partial-birth abortion ban, influences Texas’ reading of undue burden. Authored by Justice Anthony Kennedy, the opinion said the banning of one procedure could not be an undue burden because there was a legitimate government interest in protecting.

This view of undue burden reduces it to a rational basis test, Manian said, which is one of the lowest standards of proof required in court. All that is required is proof there was a valid reason for enacting the law, and the government almost always wins, she said.

“It seems no amount of burden, no matter how extreme, under this reading would pass,” Manian said. “If not this in Texas, then what? The only thing that could be struck down…would be an outright ban.”

Elizabeth Slattery, a legal fellow with the conservative Heritage Foundation, said Texas has a strong case given the environment that spawned the law, citing the case of Kermit Gosnell.

Gosnell, a doctor in Philadelphia, was charged in 2013 with seven counts of first-degree murder after it was discovered he had been delivering viable babies only to kill them. He is serving a life sentence without the possibility of parole.

However, Justice Ruth Bader Ginsburg noted that Gosnell’s clinic hadn’t been inspected for more than 15 years, while Texas law allows random clinic inspections.

Keller argued that Texas shouldn’t be limited to only responding to threats that reach the level of murder when looking at abortion safety, noting concerns about complication rates with medical abortions, which only require taking a pill.

Slattery said the Texas regulations are not a frivolous burden.

“If (abortion) is legal,” she said, “it shouldn’t result in a woman dying because a stretcher couldn’t get down a hallway.”

But the American Medical Association has declared the medical basis for the law’s provisions “groundless.”

Both Justice Samuel Alito and Kennedy questioned whether there was enough evidence supporting the claim that the Texas law caused clinics to close and asked whether the case should be sent back to a lower court for more evidence.

“There was some indication…that there may be a remand,” Manian said. “Personally, I see this as a delay tactic by the conservatives who hope delaying until there is a new justice may be better than risking a 4-4 split vote.”

Only eight justices sat for oral arguments in the Texas case following Scalia’s death last month. However, Slattery said people are making more out of Scalia’s absence than they should.

“It’s pretty common to have a handful of cases where one of the justices recuses,” she said. “It’s not unprecedented.”

With the four liberal justices widely expected to vote against Texas and the remaining three conservative justices expected vote in favor, all eyes are on Kennedy, considered the swing vote.

Manian said abortion rights groups may gather some hope from the fact Kennedy joined with the liberal wing to grant stays for enacting abortion restrictions in Texas and Louisiana.

However, Slattery predicts the Whole Woman’s Health ruling won’t set a dramatically different precedent, “(Casey’s) been the standard for more than 20 years now, and I’m not sure I see that changing dramatically with this case.”

A split court would mean the 5 th Circuit ruling stands and the Texas law would take effect; closed clinics would likely stay closed. However, the ruling wouldn’t affect cases in other jurisdictions and allows the issue to come before the Supreme Court again.

U.S. Solicitor General Donald Verrilli, Jr., who joined abortion providers in challenging the Texas law on behalf of the federal government, said the question of undue burden puts the legacy of Roe v. Wade on the line.