Six U.S. senators are calling for a federal probe into Amazon's treatment of pregnant employees at its warehouses. It's the latest push by lawmakers across the country to focus regulatory attention on the working conditions for the company's ballooning workforce.

The U.S. Equal Employment Opportunity Commission should investigate whether "Amazon systematically denies reasonable accommodations for pregnant employees at its fulfillment centers," Sen. Kirsten Gillibrand, D-N.Y., wrote in a letter co-signed by Sen. Bernie Sanders, I-Vt., Sen. Elizabeth Warren, D-Mass., and three other Democrats.

The letter, released on Friday, cited several lawsuits and at least two instances in which pregnant women accused Amazon of denying requests for reassignment or lighter duty, arguing this may have violated federal protections for workers who are pregnant or have disabilities.

In a statement late Friday, Amazon spokesperson Kelly Nantel said the company "strongly disputed" allegations of discrimination and that the two workers' accounts cited by lawmakers were not accurate as they omitted Amazon's steps to accommodate the women.

"Ensuring the health and well-being of our employees is one of our greatest responsibilities," Nantel said, noting Amazon's maternity-related benefits. "We'll keep listening to our teams and investigating any concerns they raise, and if we find that we got something wrong, we'll work hard to make it right."

Working conditions at Amazon's warehouses, which are mushrooming across the U.S., have recently attracted increased scrutiny. Amazon is now the country's second-largest private employer behind Walmart, with over 950,000 workers, most of whom staff warehouses.

Advocates have particularly focused on the speed quotas required of workers at Amazon warehouses. Critics say the pace can be unhealthy and unsustainable, forcing workers to skip bathroom breaks and skirt safety measures.

On Wednesday, California lawmakers passed a first-of-its-kind legislation that could give warehouse workers new power to fight these quotas. It would also lead to more public disclosure of specific speed demands Amazon makes of its warehouse staff and their impact on the workers' health.

Investigations by news organizations and by the labor-backed Strategic Organizing Center have found that the rate of serious injuries at Amazon warehouses has been nearly double the industry average.

Founder Jeff Bezos said in a letter to shareholders in April that Amazon has hired 6,200 safety professionals and pledged $300 million to work safety projects in 2021.

"We don't set unreasonable performance goals," he wrote. "We set achievable performance goals that take into account tenure and actual employee performance data."

Speed quotas and the company's vast automated productivity monitoring were among the key concerns of workers who pushed to unionize Amazon's warehouse in Bessemer, Ala. — a high-profile effort to form Amazon's first unionized U.S. warehouse that failed in an overwhelming vote against it.

However, Bessemer workers may get a do-over because a federal labor official has found Amazon's anti-union tactics tainted the original vote sufficiently enough to scrap its results. A regional director of the National Labor Relations Board is expected to rule in the coming weeks on whether — or when — a re-vote should take place.

Editor's note: Amazon is among NPR's financial supporters.