The United States saw the slowest population growth in a century over the past year, according to Census Bureau data released Monday.

The U.S. population grew about 1.5 million, or less than half a percent, according to the census estimates, bringing the population to 328 million.

It's the smallest gain in U.S. population since World War I. 

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Only eight states saw a birth increase, with 42 states and the District of Columbia having fewer births in 2019 than in 2018. 

The data also found that deaths outnumbered births in four states: Maine, New Hampshire, Vermont and West Virginia.

“While natural increase is the biggest contributor to the U.S. population increase, it has been slowing over the last five years,” said Dr. Sandra Johnson, a demographer/statistician in the Population Division of the Census Bureau.

“Natural increase, or when the number of births is greater than the number of deaths, dropped below 1 million in 2019 for the first time in decades.” 

A decline in international migration also contributed to the slowdown. 

International migration to the United States declined to 595,000 people in 2019, the lowest this decade, down from more than 1 million between 2015 and 2016.

The decline comes as President TrumpDonald John TrumpCDC updates website to remove dosage guidance on drug touted by Trump Trump says he'd like economy to reopen 'with a big bang' but acknowledges it may be limited Graham backs Trump, vows no money for WHO in next funding bill MORE continues to press for the building of a wall on the southern border, and as the administration takes other steps to make some forms of migration to the United States more difficult.

Changes in Puerto Rican migration since Hurricane Maria in September 2017 also shaped the trend, according to the Census Bureau, with a trend of incoming migration to the 50 states and Washington D.C. reversing between 2018 and 2019.

The Census Bureau data also indicate China has replaced Mexico as the largest sending country of foreign-born immigrants to the U.S, as of 2018, continuing a trend that began with the Great Recession at the end of the last decade. Immigration from India and China has approached or surpassed Mexican immigration levels since 2010.

Florida saw the largest population increase from international migration between the 2010 census and July 2019, with 1.1 million people, followed by California with 1.02 million and Texas with 819,000.