A new class of restrictive abortion laws, passed in recent years in a swath of states, hinges on the argument that a fetus can feel pain at 20 weeks of gestation.
But the fetal pain assertion, viewed skeptically by many scientists, hit a bump Monday when the U.S. Supreme Court declined to review a lower court ruling striking down an Arizona law that criminalized abortions at 20 weeks.
The state's ban asserted that "unborn children feel pain during an abortion at that gestational age." Federal courts last year also blocked similar "fetal pain" laws in Idaho and Georgia.
Abortion rights advocates hailed the Supreme Court's move as a signal that justices aren't inclined to take on the 40-year precedent of Roe v. Wade, which established viability at around 24 weeks (the point when a fetus is considered "viable" outside the mother's womb) and as the cutoff for most legal abortions.
"It would appear that the court is not ready or willing to deal with moving the viability line at this time," says John Robertson, chairman of the Ethics Committee of the American Society for Reproductive Rights.
"The science is weak, and it would be a major change," Robertson, a University of Texas law professor, said.
But anti-abortion activists assert that while the court's decision, offered without comment, is not the one they'd hoped for, they expect it to fuel efforts to limit abortion based on the fetal pain argument including a federal bill passed last year by the U.S. House.
"This is a disappointment, but not a major setback," says Marjorie Dannenfelser of the Susan B. Anthony List, which works to elect female candidates who oppose abortion.
She compared efforts to pass fetal pain restrictions to the "crooked path" it took to get the 2003 ban on a specific abortion procedure used late in pregnancy.
"Every single time there was a rejection, it actually built momentum toward the final goal of passage," she said.
The question of how soon fetuses can feel pain has been debated for more than three decades. Scientists, with some exceptions, have consistently argued that fetuses are not developed enough to experience pain until around the third trimester.
A 2005 analysis of numerous studies that appeared in the Journal of the American Medical Association concluded that for a fetus to feel pain its neural connections into the cortex have to be developed and that doesn't occur until sometime after the 26th week of gestation.
Robertson, the Texas bioethicist, says that the science has not changed in the past decade and there is "overwhelming consensus" around the fetal pain issue.
But Dannenfelser and other anti-abortion activists argue that support for such laws is proved by the fact that challenges have not emerged in most of the states that in recent years have criminalized abortion at 20 weeks from Alabama and Indiana to Louisiana and Oklahoma.
At the Center for Reproductive Rights, however, Director Nancy Northup has another explanation: "Some places where they've passed [fetal pain laws] there aren't even any providers in those states that do abortions after 20 weeks, so there's no way to challenge them."
You can't challenge a law, she notes, when there's no one with legal standing to mount the effort.
"It's a whole strategy to keep moving the timeline backwards," Northup says.
The fetal pain strategy has changed conversations that doctors are having with their patients, says Dr. Anne Davis, consulting medical director for Physicians for Reproductive Health and an associate professor for obstetrics and gynecology at Columbia University.
"Ten years ago, patients never asked about this," says Davis, an abortion provider. "Now we have questions from some very distraught people it's a very emotional subject, and most people aren't experts on pain physiology."
Her explanation to patients is that the science of fetal development has not changed. "The brain and the rest of the nervous system where the pain is coming from are not connected until the third trimester," she says.
But, as Davis attests, the fetal pain argument has taken root.
And Northup, at the Center for Reproductive Rights, says that there are dozens of abortion-related cases in legal pipeline, including those involving the fetal pain argument.
"The Supreme Court," she says, "is going to have the opportunity again and again and again this year and next year to see if they want to take a look again at their jurisprudence on abortion rights."