Obama administration officials estimate that most Americans will have a choice of at least five carriers when open enrollment begins in October. There are signs of increased competition, with new insurers and existing providers working harder to design more affordable and innovative plans. In 31 states, officials say there will be insurers that offer plans across state lines. The exchanges will be open to the millions of Americans who are uninsured or already buying individual coverage. Many will be eligible for federal subsidies.

But the insurance landscape will be highly varied, with some of the states that have been slow to embrace the law potentially offering the fewest options — and plans with the highest premiums — in the first year.

People in certain parts of the country may not have the robust choice of insurers that the law sought as a way to keep premiums lower and customer responsiveness high. These people are likely to have few brand-name options to choose from, and they will be gambling on plans offered by insurers new to the individual market as well as brand-new carriers. The choice of providers and costs could also vary as a result.