Proposed mega-mergers involving four of the nation’s five largest health insurance companies could leave Americans with fewer options, higher costs, and worse health care. That's according to a Consumer Reports health insurance expert in testimony before the Senate's Judiciary Committee.

“We have serious concerns that a market dominated by a few, powerful insurance companies could, ultimately, degrade the care doctors and hospitals provide to consumers,” says George Slover, a senior policy counsel at Consumers Union, the policy and mobilization arm of Consumer Reports.

Last summer, Aetna announced a $37 billion deal to buy Humana, creating a combined company that would serve 33 million customers nationwide. A few weeks later, Anthem announced it would pay $54 billion for Cigna, a move that would create the nation’s largest health insurance company, serving 53 million members. UnitedHealth Group has suggested it might try to purchase Humana if the merger with Aetna falls through.

Some in the health insurance industry claim that these mergers, such as the one between Aetna and Humana, would result in lower premiums for consumers because the companies could cut administrative costs. Plus, they argue, these health insurance mergers would give insurers more power to negotiate lower rates with hospitals and physician groups, which have recently gone through their own merger frenzy.

But Slover is skeptical. He worries that “the result will just be more market power for the big insurers and providers, at the expense of choice and value for consumers."

Research suggests that previous health insurance mergers have not worked out well for consumers. For example, a July 2015 study in the Journal of Health Economics concluded that insurance premiums tend to be higher in markets with less competition. A 2012 study also raised concerns. It found  that premiums went up after Aetna and Prudential merged in 1999.

Those at greatest risks of rates increases include millions of people with private health plans and older Americans with Medicare Advantage plans.

The proposed health insurance mergers face federal and state regulatory hurdles that could take a year or more to resolve. In particular, the U.S. Justice Department will determine whether the mergers violate antitrust laws. “We hope the Justice Department will take a hard look, and do everything in its power to keep genuine competition and consumer choice in this vital marketplace,” Slover says.

If you’re concerned about how the mergers between Aetna and Humana and Anthem and Cigna might effect your choice of health insurance companies, contact your state's insurance regulator and your state's attorney general's office.

Correction: An earlier version of this article indicated Aetna and Prudential merged in 2009.