The U.S. population grew by 0.1 percent in the past year, according to new data released by the Census Bureau, marking the lowest growth rate since the nation was founded.

The Census Bureau, in a statement on Tuesday, said the record slow rate of growth can be attributed to a decrease in net international migration — the difference between the number of people entering the country and leaving the country — a drop in fertility and increase mortality in-part because of the COVID-19 pandemic.

The U.S. population increased by 392,665 in the past year, according to the Census Bureau, making the first time since 1937 that U.S. population grew by less than one million people.

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The Census Bureau noted, however, that U.S. population growth has been on the decline for a number of years now.

“Population growth has been slowing for years because of lower birth rates and decreasing net international migration, all while mortality rates are rising due to the aging of the nation’s population,” Kristie Wilder, a demographer in the Population Division at the Census Bureau, said in a statement.

“Now, with the impact of the COVID-19 pandemic, this combination has resulted in a historically slow pace of growth,” she added.

Natural increase — which measures the number of excess births over deaths — and net international migration drove the U.S.’s population growth between July 1, 2020 and July 1, 2021, according to the Census Bureau. Natural increase accounted for 148,043 people and net international migration contributed 244,622 people. 

That period marked the first time net international migration was larger than natural increase in a given year, according to the Census Bureau.

The South was found to be the most populous region of the U.S., according to the Census Bureau, with a population of 127,225,329. The South made up 38.3 percent of the total U.S. population.

The Northeast region was the least populous in the U.S., with a population of 57,159,838 in 2021, according to the Census Bureau. That number was a 365,795 decrease from the year prior, largely because of natural decrease and a negative domestic migration, which measures the movement of people from one area in the U.S. to another.

A total of 33 U.S. states saw their populations increase between 2020 and 2021, according to the Census Bureau. The remaining 17 and the District of Columbia lost population.

Eleven of the states that lost population saw decreases greater than 10,000 people, which the Census Bureau said is a historically large number of states to lose a part of their population in one year.

Texas tracked the largest annual and cumulative numeric gain in the past year, and Idaho saw the fastest annual and cumulative population increase. New York reported the largest annual and cumulative population decline.