In early June, a little-noted bill got a historic boost. For the first time ever, Republicans sponsored a pregnancy bill, the Pregnant Workers Fairness Act, which would require the country’s employers to make small adjustments—­allowing more frequent bathroom breaks, providing a stool to sit on, switching a worker’s duties so they don’t include heavy lifting—so that pregnant employees can stay on the job.

It sounds like a small thing, and for most employers it would be. But this is the first year that two Republican senators got on board with the idea that half the workforce may at some point become pregnant.

Discrimination against pregnant employees is the clearest sign we have that the American workplace still operates under the archaic idea that men go to work and women remain in the home. Charges of pregnancy discrimination filed with the Equal Employment Opportunity Commission (EEOC) rose 65 percent between 1992 and 2007, outpacing the increase of women joining the labor force. Over the last two decades, more than 40 percent of the gender-based cases related to discriminatory hiring were about pregnancy. The most high-profile one was brought by Peggy Young against her former employer, UPS, which denied her request to be switched to light duty, something it offered disabled workers and even those whose driver’s licenses were suspended. Her case made it all the way to the Supreme Court, where this year it won a favorable ruling. Other outrageous cases abound. A nonprofit maintained a “no pregnancy in the workplace” policy. A woman says she was fired less than two weeks after she told her investment-bank employer she was pregnant. Another says she was fired on her very first day of work after she disclosed her pregnancy.

Today the majority of women who become pregnant work. The share of first-time mothers-to-be who stayed on their jobs increased from less than half in 1960 to two-thirds by 2008. And those who choose to keep working do so longer, with more than 80 percent staying on the job into the last month of their pregnancies. This likely springs from financial necessity: Women are the primary source of income for 40 percent of American households. Meanwhile, without the guarantee of paid maternity leave, many are probably squirreling away as much time off as they can cobble together for after their babies arrive.

What would women need to stay comfortably in the workforce? Among women ages 18 to 45 who had recently given birth, 71 percent needed more frequent breaks at work, 61 percent needed a schedule change or time off for the doctor, and more than half needed to change their responsibilities in order to do less heavy lifting or be able to sit more often. Yet an estimated quarter-million women are told each year they can’t have the changes they need to keep working while pregnant. Even more women simply never ask: Over 40 percent who needed more breaks never requested them, and nearly the same share who needed a change in duties never brought it up.

All of this flouts labor law, as the EEOC recently told the country’s employers, but continues nonetheless. The good news is that states have beefed up protections for pregnant workers on their own. Forty-five states ban discrimination against pregnant women, while 14 states and Washington, DC, require employers to make accommodations.

But the frequency of cases speaks volumes about how often pregnant employees have to worry about whether their health status threatens their jobs. It takes time and resources to bring a complaint against an employer; far more women are likely experiencing the same thing but are keeping silent.

Women have made up about half the workforce for a quarter-century. Their presence means the economy is 11 percent larger than it would have been if they stayed home. But bosses of both genders still haven’t wrapped their heads around what it means to employ people who might become pregnant, which means that half of their employees’ jobs could be put at risk over their ability to bear children.