A new study finds that proposed mergers among some of the largest health insurance companies in the U.S. would dampen competition in Colorado and more than a dozen other states. That could lead to increases in insurance rates. 

The American Medical Association says the the merger of Anthem and Cigna could cut competition in half in Grand Junction, Greeley and Colorado Springs. The companies are the second and third largest preferred provider organizations in the state. 

The AMA also warned that employers and patients could be charged higher than competitive premiums. It's urging federal and state regulators to take a hard look at the mergers.

Anthem released a statement saying the proposed merger will improve consumer choice and quality, and “reduce costs while improving health outcomes.”

The deal still needs government and shareholder approval.