WASHINGTON — New federal guidelines on job discrimination against pregnant workers could have a big impact on the workplace and in the courtroom.

The expanded rules adopted by the bipartisan Equal Employment Opportunity Commission make clear that any form of workplace discrimination or harassment against pregnant workers by employers is a form of sex discrimination — and illegal.

Updating its pregnancy discrimination guidelines for the first time in more than 30 years, the agency cited a “persistence of overt pregnancy discrimination, as well as the emergence of more subtle discriminatory practices.”

The guidelines spell out for the first time how the Americans With Disabilities Act applies to pregnant workers. And they emphasize that any discrimination against female workers based on past or prospective future pregnancies is also illegal.

Joan C. Williams, a law professor at the University of California’s Hastings School of Law in San Francisco, said the new guidelines issued this week can have two major impacts: steering EEOC investigators to be more sensitive to the sometimes special needs of pregnant workers and giving employment lawyers more ammunition in defending clients who were victims of such discrimination.