Aetna is calling off its public insurance exchange expansion plans for next year, as it becomes the latest big insurer to cast doubt on the future of a key element of the Affordable Care Act.

The nation’s third largest insurer said today that significant challenges faced by the state-based exchanges are forcing the company to withdraw expansion its plans and think about its future participation in the 15 states where it currently sells coverage.

Aetna was planning to expand to a few states like New Jersey and Indiana for 2017. The Hartford, Connecticut, insurer said earlier this year that it had lost more than $100 million on its exchange business last year, but company leaders said they thought the exchanges still might represent a good business opportunity.

Aetna covered about 911,000 people through the exchanges, a small slice of its customer base of nearly 23 million.

The exchanges have helped millions of people gain health coverage, many with assistance from income-based tax credits, but several prominent insurers say they are bleeding money on this business.

The nation’s largest health insurer, UnitedHealth Group, has drastically cut its presence in the exchanges, and the Blue Cross-Blue Shield insurer Anthem also recently reported a loss from the still-new business.