To understand the current case, readers have to grasp the constitutional concept of “levels of scrutiny.” It’s not as bad as it sounds. My former constitutional-law professor, Walter Dellinger, summarized it this way: “If government wants to do something to you, it has to give a reason. If it wants to do something really bad, it has to give a really good reason.”

“Scrutiny” refers to how good a reason a court must demand. The lowest “level of scrutiny” is called “rational basis,” which just means that someone who’s not insane might think the law is a good idea. Strict regulation of food trucks, for example, might protect public health; it also might reduce competition. The legislature gets to decide which approach to take without a lot of second-guessing by courts.

“Fundamental rights,” on the other hand, are subject to “strict scrutiny”—laws can’t abridge them without “a really good reason.” Roe v. Wade in 1973 held that the right to choose abortion (before the third trimester of pregnancy) was “fundamental.” Restrictions on it were justified, if at all, only by a “compelling governmental interest” (the classic “really good reason”)—and by proof that the restriction really will achieve that interest. That test is hard to pass. For two decades, most restrictions failed it.

In 1992, however, a three-justice plurality on the Court announced it was reaffirming “the essential holding” of Roe. But that case, Planned Parenthood of S.E. Pennsylvania v. Casey, didn’t really reaffirm Roe; it created a brand-new “level of scrutiny” that’s still confusing courts today.

The joint opinion by Justices Sandra Day O’Connor, Anthony Kennedy, and David Souter made clear that government is free to express “profound respect for the life of the unborn” at any stage of pregnancy. That is, the state can try to “persuade [a pregnant woman] to choose childbirth over abortion” and it can deliberately make it “more difficult or more expensive” to get an abortion—as long as its regulations don’t form a “a substantial obstacle to the woman's exercise of the right to choose.” This is called the “undue burden” standard. It means you can deliberately “burden” the right, as long as you don’t do so “unduly.”

What is an “undue burden”? The three justices in Casey said it was any measure that had “the purpose or effect of placing a substantial obstacle” in the path of a woman seeking abortion. A regulation could prevent some women from getting an abortion at all, and still not be an “undue burden.” The Casey Court approved a 24-hour waiting period, which makes abortion nearly impossible for at least some women in rural areas, and it approved an “informed consent” requirement that had little to do with medical risks and instead involved conveying a great deal of information—like the “gestational age of the fetus” and the availability of child support and adoption—aimed at making childbirth seem like a better option. The only measure the plurality disapproved was a requirement that married women in most cases notify their husbands before getting an abortion. That was an “undue burden,” the plurality said, because it would be a “substantial obstacle” in “a large fraction” of the cases to which it applied.