The guidelines say that when it comes to parental leave "similarly situated" men and women must be treated on the same terms.
The updated guidelines come two weeks after the Supreme Court agreed to take under consideration a dispute over the EEOC's duty to try to settle charges of job discrimination before filing lawsuits against employers.
The issue has gained increasing attention and had vexed business groups as the Obama administration ratchets up its enforcement of the nation's anti-discrimination laws.
"Despite much progress, we continue to see a significant number of charges alleging pregnancy discrimination, and our investigations have revealed the persistence of overt pregnancy discrimination, as well as the emergence of more subtle discriminatory practices," EEOC Chairwoman Jacqueline A. Berrien said in a statement.
The latest EEOC data shows a 46 percent increase in pregnancy-related complaints to the EEOC from 1997 to 2011.
The guidelines were last updated in 1983. The newly revised policy spells out for the first time how the Americans With Disabilities Act, as amended in 2008, might apply to pregnant workers. And it emphasizes that any discrimination against female workers based on past or prospective future pregnancies is also illegal.
The American Civil Liberties Union welcomed the update, which was approved Monday on a 3-2 partisan-line vote by the Democratic-led commission.
"Far too many pregnant workers have been forced off the job and denied a paycheck just when they need it most, despite the fact that they are willing and able to come to work," said Laura W. Murphy, director of the ACLU's Washington Legislative Office. "Pregnancy is not a justification for excluding women from jobs that they are qualified to perform, and it cannot be a basis for denying employment or treating women less favorably than co-workers."
Debra L. Ness, president of the National Partnership for Women and Families, called the new guidelines "a powerful tool in the effort to eradicate the unlawful and unequal treatment of pregnant women in the workplace," Ness said.
"This is a significant victory," said Joan C. Williams, a law professor at the University of California's Hastings School of Law in San Francisco. "Too often today pregnant workers lose their jobs when employers deny them accommodations that are regularly granted to other employees."
Commissioners Constance Barker and Victoria Lipnic dissented from the decision, saying the commission was overstepping its authority. Both were first appointed by Republican President George W. Bush.
The U.S. Chamber of Commerce has been critical of EEOC decisions during the Obama years, and the EEOC matter was no exception.
Randel Johnson, the chamber's vice president for labor issues, called it "an agency which often advances questionable enforcement tactics and legal theories."