The U.S. grew from 2018 to 2019 by almost a half percent, or about 1.5 million people, with the population standing at 328 million this year, according to population estimates.

That’s the slowest growth rate in the U.S. since 1917 to 1918, when the nation was involved in World War I, said William Frey, a senior fellow at The Brookings Institution.

For the first time in decades, natural increase — the number of births minus the number of deaths — was less than 1 million in the U.S. due to an aging population of Baby Boomers, whose oldest members entered their 70s within the past several years. As the large Boomer population continues to age, this trend is going to continue.

“Some of these things are locked into place. With the aging of the population, as the Baby Boomers move into their 70s and 80s, there are going to be higher numbers of deaths,” Frey said. “That means proportionately fewer women of child bearing age, so even if they have children, it’s still going to be less.”