WASHINGTON — Peggy Young only has to look at her younger daughter to be reminded of how long she has fought United Parcel Service over its treatment of pregnant employees, and why.

Young was pregnant with Triniti, who’s now 7 years old, when UPS told Young that she could not have a temporary assignment to avoid lifting heavy packages, as her doctor had ordered.

“They told me basically to go home and come back when I was no longer pregnant,” Young said. “I couldn’t believe it.”

She sued the Atlanta-based package-delivery company for discriminating against pregnant women. She lost two rounds in lower courts, but the Supreme Court will hear her case Wednesday.

The 42-year-old Young, who lives in Lorton, Virginia, said her persistence is not only for herself. “I am fighting for my two daughters and I’m fighting for women who want to start a family and provide for the family at the same time,” she said.

UPS spokeswoman Kara Gerhardt Ross said the law is on the company’s side. “UPS did not intentionally discriminate,” Ross said.

The outcome could have wide-ranging effects. Three-quarters of women entering the workforce today will become pregnant at least once while employed, and many will work throughout their pregnancies, employment discrimination expert Katherine Kimpel wrote in a court brief. Some will experience complications or physical effects that cause them to ask their employers for a change of duties or other modifications, she said.

Young’s case hinges on the Pregnancy Discrimination Act, a law that Congress passed in 1978 specifically to include discrimination against pregnant women as a violation of the 1964 Civil Rights Act. Congress acted after the Supreme Court, then composed entirely of men, said workplace rules that excluded pregnant workers from disability benefits and insurance coverage did not amount to sex discrimination under the landmark civil rights law.

The question in Young’s case is whether UPS violated the law through its policy of providing temporary light-duty work only to employees who had on-the-job injuries, were disabled under federal law or lost their federal driver certification. “If you were painting your house and fell off a ladder, or if you had a ski accident, that wouldn’t qualify for restricted light duty. That’s where pregnancy fell at that time. It was not covered in any state law except California’s,” Ross said.

UPS also notes in its court filings that the Postal Service maintains an identical policy when it comes to pregnant workers. The Postal Service declined comment.