Four months after the Department of Justice allowed UnitedHealth Group Inc. and PacifiCare Health Systems Inc. to merge, creating the country’s second-largest health insurer, the main trade group for U.S. doctors is calling on antitrust regulators to reverse “imbalances in the marketplace.”

In its fifth annual study of health insurance competition in local U.S. markets, the American Medical Association found that consolidation is allowing record profits for insurers but causing escalating premiums for customers. Sweeping consolidation among carriers provides physicians with few legal ways to fight downward pressure on their fees, the doctors’ group argued.