(CNN)US population growth slowed this decade, partially due to fewer births, more deaths and a decline in international migration to the US.
Newly released population estimates from the US Census Bureau show that the US population grew just 0.48% since July 2018. Growth has slowed every year since 2015.
The US population increased by 19.5 million since April 2010 -- that's about 0.66% per year. Last decade, the population grew an average of 0.97% annually.
Natural increase -- the difference between births and deaths in the US -- dropped to 957,000 in 2019 -- the first time in at least 40 years that the natural increased dropped below 1 million.
40% of the US population is concentrated in the South, the most populous region in the US and the one that saw the largest growth. Between 2018 and 2019, more than 1 million people moved into the South -- just as more people moved out of the northeastern US.
Migration between 2018 and 2019 was the lowest it's been this decade
Migration bottomed between 2018 and 2019, census estimates show. During that period, an estimated 595,000 people immigrated to the US, a notable drop from the 1,047,000 people who migrated to the US between 2015 and 2016.
Since 2010, international migration has added about 7.9 million to the US population, though that level has steadily declined since 2016.
The decline in net migration -- which measures people who move out and people who move into a country -- is largely due to a steep drop in foreign-born immigration, according to Anthony Knapp, a demographer with the Net International Migration Branch of the census.
The decline coincided with a "relatively small but steady rise" in foreign-born emigration, he said. The Census Bureau defines "foreign-born" as people who are not US citizens at birth.
"No specific country stood out in terms of declining immigration," Knapp said.
In 2018, China surpassed Mexico to become the country from which the most foreign-born immigrants moved to the US.
Mexico immigration has sharply dropped after a brief peak in 2011, census estimates show. Immigration from Canada remains largely unchanged at around 50,000 every year.
Fewer people are immigrating to the US
The changes in migration since 2015 reflect three major trends, Knapp said: Fewer foreign-born people are immigrating to the US, more foreign-born people are emigrating from the US and Puerto Ricans who migrated to the US mainland following Hurricane Maria in 2017 are now moving back to the territory in higher numbers.
Net in-migration from Puerto Rico to the 50 US states and Washington increased to 123,000 between July 2017 to June 2018, including the month when the hurricane made landfall. But in the months that followed until June 2019, the trend reversed, and 8,000 people returned to Puerto Rico.