Self-insurance was already growing before Mr. Obama signed the law in 2010, making it difficult to know whether the law is responsible for any recent changes. A study by the nonpartisan Employee Benefit Research Institute found that about 59 percent of private sector workers with health coverage were in self-insured plans in 2011, up from 41 percent in 1998.

But experts say the law makes self-insurance more attractive for smaller employers. When companies are self-insured, they assume most of the financial risk of providing health benefits to employees. Instead of paying premiums to insurers, they pay claims filed by employees and health care providers. To avoid huge losses, they often sign up for a special kind of “stop loss” insurance that protects them against very large or unexpected claims, say $50,000 or $100,000 a person…

Insurance regulators worry that commercial insurers — and the insurance exchanges being set up in every state to offer a range of plan options to consumers — will be left with disproportionate numbers of older, sicker people who are more expensive to insure.

That, in turn, could drive up premiums for uninsured people seeking coverage in the exchanges. Since the federal government will subsidize that coverage, it, too, could face higher costs, as would some employees and employers in the traditional insurance market.