PHOENIX — Three disability-rights groups are urging the U.S. Supreme Court to let Arizona enforce its ban on abortions at 20 weeks as a necessary safeguard against “postnatal eugenics.”

A lawyer for the groups acknowledges the 2012 law deals only with abortion. And with Arizona law already barring post-viability abortions, the law effectively deals with only a narrow window of three to four weeks.

Nikolas Nikas, an attorney for the Bioethics Defense Fund , which is leading the effort, told the justices much more is at stake.

He said part of the justification for abortions at or beyond 20 weeks is that certain abnormalities cannot be ascertained before that point. And that makes such abortions less about unintended pregnancies and more about the child in the womb, he argued.

“Aborting children with disabilities is a form of discrimination that threatens to devalue the lives of people born and living with disabilities,” Nikas wrote. And he said that is a bad precedent that the Arizona law is designed to prevent.

Nikas said the Arizona law simply reflects both federal statutes and “emerging social norms that reject discrimination” against those with disabilities.

The legal brief represents an new front in the bid to allow Arizona to do what the U.S. Supreme Court has not permitted any state to do: move up the point at which a woman’s right to terminate a pregnancy can be trumped by the state’s interest. To date, the justices have pretty much set that line at viability — the point a fetus can live outside the womb.

Maricopa County Attorney Bill Montgomery, who is leading the defense of the Arizona law, is concentrating on two issues: higher risk to women for abortions beyond 20 weeks and disputed evidence a fetus at that point can feel pain. But the 9th U.S. Circuit Court of Appeals rejected that, citing Supreme Court rulings giving women an absolute right to decide prior to viability.

Nikas, however, told the justices those precedents do not matter in this case. “This court has never recognized a right to abort children identified as having disabilities, and it should not recognize such a right,” he said.

Nikas said one of the Arizona doctors who challenged the law testified in court that about 70 percent of his patients seeking abortions at or after 20 weeks do so due a “serious or lethal fetal anomaly.” Nikas said while some involve diagnoses likely to be fatal, many involved nonfatal conditions like Down syndrome, cystic fibrosis and spina bifida.

“Such prenatal screening for disabilities has resulted in widespread prenatal termination of many classes of people with disabilities,” he wrote.

Foes of the law, represented by the Center for Reproductive Rights, have not yet responded to the legal brief.

However, spokeswoman Kate Bernyk pointed out that the organization has fought — and the high court has rejected — similar efforts to do an end-run around the rights of women to make decisions prior to viability.

And Nancy Northup, the organization’s president, said the issue comes down to letting women decide “based on the complicated individual and personal circumstances they face.”