Last month, the U.S. workforce lost 865,000 women—and that might be just the tip of the iceberg.

According to the 2020 Women in the Workplace report by LeanIn.Org and McKinsey & Company, one in four women in corporate America are now considering downshifting their jobs or leaving the workforce. That number holds steady even for the country’s senior-most women leaders.

Senior-level women—executives at the level of vice president and up—have dedicated a great deal to their careers over their lifetimes, working to earn promotions and run teams, often in workplaces that are structurally biased against women. They’re leaders. They’re trailblazers. So why are so many thinking about leaving?

Lack of access to childcare is a key factor driving millions of women from the broader U.S. workforce. Executive women likely have better access to childcare than other workers, but their responsibilities have still skyrocketed at home. Many men at their level have spouses who handle childcare and housekeeping, but most senior-level women don’t have that kind of support. Among senior leaders with partners, 63% of women have partners who work full-time, compared to just 35% of men. And mothers are three times more likely than fathers to be handling all of their family’s housework and childcare.

But senior-level women without children are struggling, too. They report feeling more stressed, more exhausted, and more burned out than the men who are their peers. At Lean In, we’re hearing stories from executive women who say that the emotional labor needed to get employees through this crisis is disproportionately falling to them. Some have been asked by the men they work with to step in to check in on employees and lead difficult internal conversations. As one senior vice president at a private equity firm told us, “I’m dealing with multiple people in crisis at the same time. Some people’s parents or grandparents died of COVID or got sick. That’s a very different conversation to the one you have with a mother homeschooling with children.” This emotional labor can be draining and often goes unnoticed and unrewarded.

Even in the best of times, being a woman leader comes with extra challenges. Research shows that women are often held to higher performance standards and blamed more for failures. Senior-level women are often the only woman in the room at work, and “Only” women are more likely to have their judgment questioned and need to provide more evidence of their competence. Imagine what it’s like for women in leadership when the stakes are as high as they are now, with leaders writing the playbook as they go. It’s no wonder senior-level women feel pressured to work more than senior-level men; they’re facing harsher judgment overall.

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Senior-level women—tough, tested, ambitious leaders—are being pushed to their limits and beyond. This is a real problem for companies because these leaders are too important to lose. Companies with more women in senior leadership perform better and have better cultures. Executive women are more likely than men to raise workplace issues that matter to employees, including work/life flexibility and equal pay. They’re also more likely to be mentors and sponsors to other women—and to be allies to women of color.

It’s critical that companies double down on supporting senior-level women right now. This means rethinking goals and expectations set before the pandemic while it’s harder than ever for employees to live up to “business as usual” expectations. It also means making it OK for women in leadership—and all leaders—to set boundaries to protect their families or personal time. And executive men should ask themselves: Am I doing enough to support the women at my level, especially those with more demands at home? Is there more I can do to take on emotional labor at work?

Unless companies take swift action, we risk losing hard-won gains in the number of women in leadership. The ripple effects will spread across organizations: companies will lose women’s experience and expertise, and other women will lose their greatest champions. Retaining senior-level women leads to better business results, better work cultures, and more support for women. In an unprecedented time full of hard choices for companies, this one is simple.

Sheryl Sandberg is Facebook Inc.’s chief operating officer and the founder of LeanIn.Org. Rachel Thomas is the CEO of LeanIn.Org.