More than 120 Democratic lawmakers in Congress, women’s and civil rights advocates, women business leaders and a bipartisan group of state legislators filed friend-of-the-court briefs in the Supreme Court on Thursday in support of Peggy Young, a former UPS driver, and her pursuit of reasonable accommodations for pregnant workers to keep them on the job.

The American Civil Liberties Union and the nonprofit A Better Balance, which advocates for better policies to support working families, argued in a brief that pregnant workers are the only category of worker routinely denied accommodations, like light duty work, stools, water bottles and bathroom breaks, in order to be able to continue working. The Women’s Chamber of Commerce argued in its brief that providing accommodations to pregnant workers is important to the national economy.

And members of Congress argued that lower courts have misinterpreted a 1978 Pregnancy Discrimination Act that was designed expressly to protect pregnant workers from discrimination on the job.

The Supreme Court case centers on Peggy Young, 42, a former UPS driver in Landover, Md., who requested light duty, on her doctor’s orders, rather than continue to lift heavy packages as her pregnancy advanced. UPS refused, saying company policy was to give light duty or accommodations to workers injured on the job, or those who had lost drivers’ licenses — not to pregnant workers. She sued UPS in federal court. Two lower courts, including the 4th Circuit Court of Appeals in Richmond, decided in UPS’ favor, that the policy was “pregnancy neutral.”

“We think incorrect interpretation of the law: Under their interpretation, pregnant workers are pretty much the only people who don’t get accommodations,” said Ariela Migdal, senior staff attorney for the ACLU’s Women’s Rights Project. “The only people being pushed out of their jobs and forced onto unpaid leave are going to be pregnant women. It’s really sex discrimination that’s at stake.”

Migdal said the ACLU is representing other women who, like Young, were denied reasonable accommodations and instead were forced to take unpaid leave or lost jobs. Many women have lost housing, health insurance and forced to rely on public assistance after being forced off jobs, she said.

“We shouldn’t have to debate this issue in the 21st century,” Sen. Bob Casey, D-Pennsylvania, said in an email. “But the facts of this case make clear that far too many pregnant women suffer workplace discrimination.”

Casey and many of the lawmakers signing onto the brief also are pushing a Pregnant Workers Fairness Act that would ensure employers give pregnant workers reasonable accommodations to stay on the job. According to a growing number of claims with the Equal Employment Opportunity Commission, many pregnant workers have been forced to take unpaid leave, or were fired rather than be given accommodations that are routinely given to other workers.

The bill, which President Obama has urged lawmakers to sign, has languished without a single Republican co-sponsor. Casey said the number of pregnant workers in a given year is a very small share of the workforce — about 1.6 percent — and that the accommodations many may need in order to keep working are often minimal.

“These accommodations aren’t that difficult,” Casey said in an interview. “The challenge employers face to provide them is negligible. If we could pass accommodations for workers with disabilities with overwhelming bipartisan support, why not for pregnant workers?”

A growing number of state legislatures have unanimously, or with bipartisan effort, passed pregnant workers’ fairness laws to ensure reasonable accommodations for pregnant workers.

UPS stands by its policy. “UPS is committed to a fair workplace and has consistent policies to prevent discrimination and adhere to our legal obligations,” UPS spokeswoman Susan Rosenberg said in an email. “This was validated by both the district and appellate courts.”

Friend-of-the-court briefs for Young were due to the Supreme Court on Thursday. Amicus briefs for UPS are due in October, UPS officials said. The Supreme Court will hear oral arguments Dec. 3.