Mississippi Gov. Phil Bryant (R) on Monday signed a law—HB 1510—that bans abortion after 15 weeks’ gestation. The law went into effect immediately, giving Mississippi the honor of having the most stringent abortion ban in the United States.

Bryant tweeted that he was “committed to making Mississippi the safest place in America” for fetuses.

By Tuesday afternoon, however, the law was blocked by a federal judge.

Within hours of Bryant signing the law, the Center for Reproductive Rights filed a lawsuit on behalf of Jackson Women’s Health Organization—Mississippi’s sole abortion clinic—challenging the law as unconstitutional and seeking a temporary restraining order to block it.

“Under decades of United States Supreme Court precedent, the state of Mississippi cannot ban abortion prior to viability, regardless of what exceptions are provided to the ban,” the complaint reads.

“Accordingly, the ban is unconstitutional as applied to all women seeking pre-viability abortions after 15 weeks.”

U.S. District Court Judge Carlton Reeves granted plaintiffs’ temporary restraining order on Tuesday afternoon in a brief two-page order. “The Supreme Court says every woman has a constitutional right to ‘personal privacy’ regarding her body. That right protects her choice ‘to have an abortion before viability,’” Reeves wrote, citing Roe v. Wade and Planned Parenthood v. Casey.

“The plaintiffs are substantially likely to succeed on their claim that HB 1510 is unconstitutional. The law threatens immediate, irreparable harm to Mississippians’ abilities to control their ‘destiny and … body,’” Reeves wrote, citing Whole Woman’s Health v. Hellerstedt. “This is especially true for one woman scheduled to have a 15-week abortion this afternoon.”

The cases Reeves cited—Roe, Casey, and Hellerstedt—all prohibit a state from interfering with a pregnant person’s right to a pre-viability abortion. Medical consensus is that a fetus is not viable until about 24 weeks. Anti-choice activists have attempted to roll back the point of viability, arguing that some fetuses can survive outside the womb at 22 weeks.

But no fetus is viable at 15 weeks, as the plaintiffs note in their complaint.

“HB 1510 places viability at 15 weeks—about two months earlier than where the medical consensus places it,” Reeves wrote in his order, noting that the law is of “dubious constitutionality.”

Dubious constitutionality is putting it lightly. Anti-choice politicians have been laser-focused on enacting pre-viability abortion bans—a 20-week ban here, a six-week ban there—but all of them suffer the same problem: They’re wildly unconstitutional.

These abortion bans have been unconstitutional since Roe v. Wade legalized abortion in 1973, and they will remain unconstitutional unless the U.S. Supreme Court either overturns its landmark decision or reverses 40 years’ worth of case law about the importance of fetal viability. And given the Court’s 2016 Hellerstedt decision, that’s not likely to happen unless President Trump is able to stack the Court with conservative judges.

For now, pre-viability abortion bans like Mississippi’s are unconstitutional.