The market for health-insurance in the U.S. is already so highly concentrated that pending tie-ups among four of the country’s largest insurers risk hurting both consumers and doctors, the American Medical Association said.

Anthem Inc.’s proposed takeover of Cigna Corp. and Aetna Inc.’s bid for Humana Inc. would reduce competition among insurers in 154 metropolitan areas, worsening already concentrated markets, the organization said in studies released Tuesday. The mergers could cause premiums to go up and decrease payments to doctors, the AMA said.

"The prospect of reducing five national health insurance carriers to just three should be viewed in the context of the unprecedented lack of competition that already exists in most health insurance markets," the AMA said.

‘Limited Overlap’

The takeover would make Anthem the largest health insurer in the U.S. by members and give it more scale in commercial coverage.

Anthem and Cigna have "limited overlap in a highly competitive industry," said Kristin Binns, a spokeswoman for Anthem. By combining, they will be able to operate more efficiently and reduce costs, she said.

"Our commitment to ensuring that consumers have expanded access to high-quality, affordable health coverage is the foundation of the proposed transaction and will remain Anthem’s top priority," Binns said.

Spokesmen for Aetna, Cigna and Humana didn’t respond to requests seeking comment.

Assistant Attorney General Bill Baer, who leads the Justice Department’s antitrust division, which is reviewing the surge of deals, has said he will assess the industry as a whole to make sure competition is preserved and the mergers don’t lead to higher costs.

Antitrust reviews of health-insurer deals typically focus on competition in local markets. In the past, companies have been able to resolve government concerns by selling parts of their business to competing insurers. If the Justice Department changes its approach to insurer reviews and considers a national market for health insurance, that could pose a bigger risk for the transactions, according to Bloomberg Intelligence analyst Jennifer Rie.