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."

Four states had a natural decrease, where deaths outnumbered births: West Virginia, Maine, New Hampshire and Vermont.

For the first time this decade, Puerto Rico had a population increase. The island, battered by economic stagnation and Hurricane Maria in the past several years, increased by 340 people between 2018 and 2019, with people moving to the island offsetting natural decrease.

International migration to the U.S. decreased to 595,000 people from 2018 to 2019, dropping from as many as 1 million international migrants in 2016, according to the population estimates. Immigration restrictions by the Trump administration combined with a perception that the U.S. has fewer economic opportunities than it did before the recession a decade ago contributed to the decline, Frey said.

"Immigration is a wildcard in that it is something we can do something about," Frey said. "Immigrants tend to be younger and have children, and they can make a population younger."

Ten states had population declines in the past year. They included New York, which lost almost 77,000 people; Illinois, which lost almost 51,000 residents; West Virginia, which lost more than 12,000 people; Louisiana, which lost almost 11,000 residents; and Connecticut, which lost 6,200 people. Mississippi, Hawaii, New Jersey, Alaska and Vermont each lost less than 5,000 residents.

Regionally, the South saw the greatest population growth from 2018 to 2019, increasing 0.8% due to natural increase and people moving from others parts of the country. The Northeast had a population decrease for the first time this decade, declining 0.1% due primarily to people moving away.

Monday's population estimates also offer a preview of which states may gain or lose congressional seats from next year's apportionment process using figures from the 2020 Census. The process divvies up the 435 U.S. House seats among the 50 states based on population.

Several forecasts predict California, the nation's most populous state with 39.5 million residents, losing a seat for the first time. Texas, the nation's second most-populous state with 28.9 million residents, is expected to gain as many as three seats, the most of any state.

According to Frey's projections on Monday, Florida stands to gain two seats, while Arizona, Colorado, Montana, North Carolina and Oregon each stand to gain a seat. Besides, California, other states that will likely lose a seat are Alabama, Illinois, Michigan, Minnesota, New York, Ohio, Pennsylvania, Rhode Island and West Virginia.