As the only industrialized nation in the world with no federal policy mandating paid maternity leave, the United States has a long and complicated history of how employers have handled pregnancy in the workplace.

From mandating reduced hours for pregnant employees to waffling on whether or not to treat pregnancy as a disability, employers have varied widely in how they have accommodated—or not accommodated—expecting women on their payrolls.

During the Progressive Era, champions of women-only protective laws in the workplace referred not only to women’s relative physical weakness, but also to their child-bearing role as mothers. In a landmark decision in 1908, the U.S. Supreme Court unanimously upheld an Oregon law limiting working hours for women, stating that because “healthy mothers are essential to vigorous offspring, the physical well-being of woman becomes an object of public interest and care in order to preserve the strength and vigor of the race.”

Passage of Title VII of the Civil Rights Act of 1964, which prohibits employers from discriminating against employees on the basis of sex, race, color, national origin and religion, put an end to such protective discrimination against women in the workplace. But it didn’t resolve the issue of how employers should handle pregnancy.

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As Susan Kelemen Gardin and Gary A. Richwald recounted in their 1986 article in the Journal of Public Health Policy, the Equal Employment Opportunity Commission (EEOC) initially determined that maternity wasn’t subject to the same rules as other temporary disabilities under Title VII. Later, the commission argued exactly the opposite: Employers were required to treat pregnancy like any other temporary disability.

This confusion over how employers should treat pregnant workers proceeded to play out in a series of legal battles. Most importantly, in the 1976 case General Electric (GE) v. Gilbert, the Supreme Court determined that an employer-provided plan paying workers part of their wages for short-term disability, but not pregnancy, did not discriminate against women. Employers shouldn’t be required to cover an “additional risk, unique for women,” the Court ruled, adding that pregnancy was “voluntary,” unlike other temporary disabilities.

What the majority decision ignored, as Gardin and Richwald noted, was that GE at the time did not exclude voluntary injuries like cosmetic surgery or attempted suicide, and that it did insure workers against male-only risks, including vasectomies.

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In 1978, in response to the controversial GE decision and pressure from women’s rights advocates, Congress passed the Pregnancy Discrimination Act as an amendment to Title VII of the Civil Rights Act. The law made it illegal for companies with 15 or more employees to consider pregnancy in hiring, firing or promotion decisions, and required employers to treat pregnant workers the same as non-pregnant workers who are “similar in their ability or non-ability to work.”

Pregnancy discrimination did not end with passage of the new law, however. In court, employers charged with such discrimination were able to successfully argue that workers who got pregnant were similar to workers who were injured off the job, and did not deserve special accommodations.

When Peggy Young, a delivery driver for United Parcel Service (UPS) in Maryland, became pregnant in 2006, her midwife recommended that she not lift over 20 pounds during her pregnancy. Her supervisors turned down her request for lighter duty, and Young was forced to take unpaid leave, during which she lost her health insurance. Young later sued UPS, claiming the company had violated the Pregnancy Discrimination Act by failing to provide her with the same accomodations as other non-pregnant employees.

Two lower courts had ruled in favor of UPS, agreeing that the company had followed a “pregnancy-blind” policy by allowing light-duty work only to workers who were injured on the job, had a condition covered under the Americans with Disabilities Act or lost their Department of Transportation certifications. In front of the Supreme Court, Young’s lawyer pointed out that UPS did give accommodation to workers who were impaired while not on the job, including ones with drunk-driving convictions.

In 2015, the Court ruled 6 to 3 in Young’s favor, but it didn’t establish outright protection for expectant mothers. The majority decision only stated that if employers are accommodating big groups of other workers (such as workers with disabilities) but not pregnant women, they are probably violating the Pregnancy Discrimination Act.

Despite passage of the 1978 law, Young’s qualified victory (she reached a settlement with UPS later in 2015) and efforts by many big companies to expand their parental leave policies and other maternity-related benefits (such as lactation rooms), claims of pregnancy-related discrimination in the United States have only increased over the past two decades.

In 2018, 2,790 federal cases alleging pregnancy discrimination were filed with the EEOC. Gillian Thomas of the ACLU’s Women’s Rights Project told CNBC that discrimination is even more widespread than that, especially among low-wage workers and women of color. Thomas cited a report by the National Partnership on Women and Families, which estimated that some 250,000 pregnant workers each year do not get the accommodations to which they are entitled.

Some federal lawmakers have proposed a new Pregnant Workers Fairness Act, which would explicitly require employers to make reasonable accommodations for pregnant workers, rather than simply require accommodation if someone else at the same company is already receiving it. Congress has yet to hear debate on such legislation, similar forms of which have been proposed every year since 2012.

Meanwhile, as more than 80 percent of American women become mothers at some point in their life, and many of them have jobs when they do, the issue of how employers treat pregnant workers is likely to remain in the spotlight.