If the "total fertility rate" drops below 2.1 children per woman, a country's population will shrink unless there are compensating levels of immigration.
And that's what's been happening in the U.S. since at least 2008. Our total fertility rate has dropped below replacement levels, but has been masked by high levels of immigration in..."
http://www.lifenews.com/2016/08/19/americas-looming-demographic-winter-our-birth-rate-is-so-low-its-a-national-emergency/
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