WASHINGTON — More than 120 members of Congress urged the Supreme Court on Thursday to recognize that pregnant workers are entitled to reasonable accommodations such as light duty, saying it’s needed to ensure that expecting mothers are not forced out of their jobs.

In a friend-of-the-court brief, the Democratic lawmakers — 99 from the House, including Minority Leader Nancy Pelosi, and 24 senators — said UPS delivery driver Peggy Young of Lorton, Virginia, was unfairly treated by her employer when it asked her to take unpaid maternity leave rather than provide a less strenuous position as her doctors advised.

Many of the lawmakers are pushing legislation to make the pregnancy protections explicit in federal law. They argue that the lower appeals court in Richmond, Virginia, ruled incorrectly by siding with UPS.

Young, 42, should have received the light work accommodation because current federal law provisions are designed to “ensure that pregnant women were no longer treated as second-class citizens on the job,” they wrote.

At issue is the 1978 Pregnancy Discrimination Act, which bars employers from discriminating based on pregnancy, childbirth or related conditions. The law requires that pregnant workers be treated at least as well as other employees similar in their ability or inability to work, but is silent as to whether it specifically requires reasonable accommodations for pregnancy.

UPS says its accommodation policies are “pregnancy-neutral,” allowing light-duty assignments only in cases in which employees were injured on the job, had a disability as defined by the Americans With Disabilities Act or whose injuries made them ineligible for commercial driver’s licenses.

The case comes as women now make up nearly half of the U.S. workforce. Of the women entering the workforce, three-quarters will be pregnant and employed at some point, according to the lawmakers.

President Barack Obama has called on Congress to pass the lawmakers’ bill, led by Sens. Bob Casey, D-Pa., Jeanne Shaheen, D-N.H., and Rep. Jerrold Nadler, D-N.Y., to make the pregnancy protections unmistakable and clear. In July, the Equal Employment Opportunity Commission also issued new guidelines that prohibit employers from forcing pregnant workers to take leave and acknowledge that employers may have to provide light duty for pregnant workers.

Emily Martin, general counsel for the National Women’s Law Center, said the lower court ruling could especially hurt women in low-wage jobs.

“Women are being asked to choose between the health of their pregnancy and their paycheck, and that’s a choice they shouldn’t have to make,” she said.

The Supreme Court case is Young v. United Parcel Service, 12-1226. Oral arguments are scheduled for Dec. 3.