There's a lot of discussion today about fertility and population growth, but much of it reflects the common presumption that when the Total Fertility Rate drops down the replacement level of about 2.1, then the population stops growing.

Even leaving aside immigration, this is wrong because of the phenomenon of Population Momentum. As I wrote in VDARE.com in 2006

"A population will typically grow for 50-60 years after reaching replacement level fertility, 

"Population momentum" is a little complicated to explain, but try thinking of it from a grandparent's perspective. Imagine two neighbors comparing notes on who has more grandchildren. The one who lives on the north side of the street says, "My children each have two children in their families." 

The neighbor who lives on the south side of the street replies, "So do mine." 

The northern neighbor says, "Then you must have four grandchildren, just like me." 

The southern neighbor laughs, "No, I have eight grandchildren! See, you only had two children, so you have four grandchildren. But I had four children, so I have eight grandchildren.