WASHINGTON -- The landmark health care reform law known as Obamacare appears to be succeeding in its twin goals of extending health coverage to the uninsured and enabling people to access life-saving treatments.

But this humanitarian success also underscores the newness and fragility of the remade health insurance market, as a new report issued Wednesday by the Blue Cross Blue Shield Association shows. The customers who flocked to the exchanges are sick and are using a lot of medical care, a trend that could jeopardize Obamacare's gains by destabilizing the health insurance system .

Greater access to health care for people with pre-existing conditions who were shut out of the old market and those whose low incomes made health insurance too expensive before the Affordable Care Act's subsidies became available in 2014 was one of the core goals of the law President Barack Obama enacted six years ago.

And health insurance exchanges are growing, with nearly 13 million enrolled as of Jan. 31, when the most recent sign-up period ended. Subsidized coverage from these marketplaces and the expansion of Medicaid under Obamacare in 31 states and the District of Columbia are the main reasons why an estimated 20 million more people have health insurance this year than before the Affordable Care Act became law and the uninsured rate declined to a historic low .

But the sicker population making up these exchange marketplaces doesn't seem to be getting balanced out by healthier customers who pay into the insurance risk pool without drawing down as much in benefits. Absent that balance, health insurance companies will raise rates further in future years to cover their costs, which could price out consumers -- especially those who are healthy and who need coverage least and thus are the least costly to insure.