If Peggy Young, who was a driver for United Parcel Service, had had an accident that limited her ability to lift heavy packages, or even lost her license because of driving while intoxicated, U.P.S. would have allowed her to go on “light duty” or assigned her another type of work. But Ms. Young got pregnant. When her doctors told her not to lift packages over 20 pounds to avoid jeopardizing the pregnancy, U.P.S. refused to accommodate her and effectively compelled her to go on unpaid medical leave.

Her case, which has implications for millions of American women and their families, will be argued before the Supreme Court on Wednesday. It is an opportunity for the court to strike a blow against discriminatory treatment and the resulting economic harm that are too often imposed on women who get pregnant — as the vast majority of women entering the work force eventually do.

Although many women can work through an entire pregnancy without job modifications, some — especially those in low-wage jobs requiring long hours, prolonged standing and heavy lifting — may require temporary help to safeguard their own health and their pregnancies.

U.P.S. claims it has a legal right to deny pregnant workers who have temporary physical limitations the flexibility it shows workers with other conditions that similarly affect the tasks they are able to perform. It said its collective bargaining agreement limited work modifications to only three categories: those with injuries that occur on the job; people covered by the Americans With Disabilities Act; and those who lose their Department of Transportation certification because of a legal impediment, like a license revoked for driving while intoxicated. Sorry, pregnancy is not included.