The number of people in the U.S. without health insurance increased in 2017 for the first time since Obamacare took effect, a possible sign that Obamacare's rising premiums are putting insurance out of reach for millions of people.

The number of uninsured rose by 3.2 million from 2016 to 2017, according to the latest Gallup-Sharecare Well-Being Index.

That increase means 12.2 percent of the U.S. population was uninsured last year. In contrast, 10.9 percent was uninsured in 2016, a record low since Gallup and Sharecare started tracking the rate in 2008.

The 2017 uninsured rate is still lower than the peak uninsured rate of 18 percent in 2013, before people became able to buy government-subsidized healthcare plans through exchanges or enroll in expanded Medicaid, both provisions of Obamacare.

But it still represents a significant increase, one that hit blacks hardest. The index said the uninsured rate rose 2.3 percent for blacks, 2.2 percent for Hispanics, and 2 percent for young adults aged 18 to 25.

One factor in the rising uninsured rate could be the rising price of Obamacare plans. Though most who are enrolled through the exchanges receive a subsidy that shields them from price increases, those without subsidies and others who buy plans with Obamacare's mandates, often off-exchange, have to pay the full price of increases.

The Kaiser Family Foundation estimates there were 6.7 million people with high enough incomes to disqualify them for subsidies, which are given to people up to 400 percent of the federal poverty level.

Additionally, several insurers exited the exchanges in 2017, which reduced competition.

"This may have caused some Americans, especially those who failed to qualify for federal subsidies, to forgo insurance," the Gallup-Sharecare report said.

It also hypothesized that news coverage of Obamacare's repeal and replace efforts may have confused people about whether Obamacare's individual mandate obligating people buy insurance or pay a fine would be enforced.

Researchers arrived at their results by conducting more than 25,000 interviews with adults from Oct. 1 to Dec. 31.