Major declines in international immigration have contributed to a decline in growth for the U.S. population, according to a report Monday from the Census Bureau.

The nation's population grew by less than 0.5 percent in 2019 — part of a steady decline since 2015. Experts say they believe the decline stems from a lack of migrants' entering the country, in hand with a drop in so-called natural increase, which is the difference between births and deaths.

U.S. Census Bureau

The 0.48 percent increase to 328.2 million marks the slowest growth rate in the United States since 1917 to 1918, when the nation was involved in World War I, said William Frey, a senior fellow at the Brookings Institution.

"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 childbearing age, so even if they have children, it's still going to be less."

Natural population increase accounted for only 957,000 people from 2018 to 2019, which is the first time the marker has fallen short of 1 million in four decades, according to the Census Bureau.

Immigration has been at a steady decline since 2016, when 1 million migrants were added to the population. In 2019, immigration added only 595,000 people to the population.

Census experts said national patterns do not necessarily hold true for all regions.

The South, home to 40 percent the population, had the most regional growth, at 1 million people, prompted by both natural increase and domestic migration. Meanwhile, the Northeast lost 64,000 residents.

Monday's population estimates also preview which states could gain or lose congressional seats from next year's apportionment process using figures from the 2020 census. The process divvies up the 435 seats in the House of Representatives among the 50 states based on population.

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Several forecasts predict that California, the nation's most populous state, with 39.5 million residents, will lose 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 stand to gain a seat apiece.

Besides California, other states that will likely lose a seat are Alabama, Illinois, Michigan, Minnesota, New York, Ohio, Pennsylvania, Rhode Island and West Virginia.