(CNN) The number of uninsured children ballooned by more than 400,000 between 2016 and 2018, an unprecedented decline in health coverage for the youngest Americans, a new study has found.

Roughly 4.1 million children were uninsured in 2018, up from a low of 3.6 million in 2016, according to the Georgetown University Center for Children and Families, citing US Census Bureau data. Their uninsured rate jumped to 5.2% last year, up from 4.7% in 2016.

Another Census study shows a similar increase, with the uninsured rate for kids increasing to 5.5% last year, up from 5.0% two years earlier.

The trend is particularly troubling because it comes during a period of economic growth when more Americans are gaining employment, said Joan Alker, the center's executive director. She fears even more children will lose coverage if the economy falters.

"Much of the gains in children's coverage that came about as a result of the Affordable Care Act have now been reversed," Alker said.