Almost a quarter of the nation’s children under 5 are at risk of not being counted in the 2020 census, which could have serious implications for the well-being of children around the United States, according to a recent study by the Annie E. Casey Foundation.

The problem has grown worse over the last four decades, experts said. In 2010, the census failed to count nearly 1 million children younger than 5. Experts warn that it could exceed that number in 2020.