New York (CNN) For the first time in nearly a decade, the number of uninsured children in America has grown.

The reversal is unprecedented, particularly given a strong economy in which more people are landing jobs and gaining access to employer health coverage. It also comes at a time when the nation's overall uninsured rate remained flat.

Roughly 276,000 more children were uninsured in 2017 than the year before, bringing the total to more than 3.9 million, according to a report released Thursday by Georgetown University's Center for Children and Families. Some 5% of those 18 and under had no health insurance, up from 4.7% a year earlier.

"With an improving economy and a very low unemployment rate, the fact that our nation is going backwards on children's health coverage is very troubling," said Joan Alker, the center's executive director. "Without serious efforts to get back on track, the decline in coverage is likely to continue in 2018 and may in fact get worse for America's children."

What's behind the increase in the uninsured isn't clear, but Alker attributes it to the Trump administration and Congress' attempts to dismantle the Affordable Care Act and shrink Medicaid. Also, it took months for Congress to approve long-term funding for the Children's Health Insurance Program, forcing several states to tell parents they might have to shutter their programs.

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