# US population growth: "natural increase" vs "fertility rate"

Based on the 2017 US Census population projections through 2060, specifically table 1 (Projected population size and births, deaths, and migration), the "natural increase," which is US births - US deaths, was positive in 2017 at over 1 million more births than deaths, and was projected to remain positive through 2060, reaching a minimum of 400,000 in 2050 before beginning to increase again.

On the other hand, the fertility rate per woman for the US was 1.8 in 2017, according to the World Bank. This is well below the replacement rate of 2.1.

How is it possible that the US could have over 1 million more births than deaths, and that a positive natural increase would be projected to continue for 40 years, but be well below replacement fertility rate?

This question is related, but the answers there indicate that it would be temporary situation due to population momentum. However, here the Census Bureau is predicting it will continue for 40 years.

The “natural increase” in population, defined as births in the U.S. minus deaths in the U.S., in the spreadsheet you linked is affected by immigration.

Definition of TFR...

“sum of age-specific fertility rates (usually referring to women aged 15 to 49 years)”

The TFR in the U.S. in 2017 was 1.7.

The 2017 TFR was calculated from age-specific fertility rates of women born between 1968 and 2002 or a similar period. Since it uses historical data we do not need to question it. We can be critical of how it can be applied as a guide to the future if we so choose.

The under the same method births in 2060 depend upon women born between 2011 and 2045 and their TFR is unknown. Suppose we assume it will still be 1.7 because suppose we want to assume a huge change such as between 1960 and 1976 will not happen again. Births in 2060 will also depend upon immigration of women born between 2011 and 2045 and we only have data on 2011 through 2023. That means births in 2060 depends upon unknown amounts of future immigration between 2023 and 2045. A 30 year old immigrant today was born in 1993 so she won’t directly affect births in 2060. However, she can potentially give birth to a female in the next several years and that child will likely be in the child-bearing age range in 2060. (Immigrants often apply for family re-unification so some children might arrive later and about half of those children are female. An infant female arriving today will be 37 in 2060 and that is in the child-bearing age range.)

Deaths in 2060 depends upon births around the year 1980, whether born in the U.S. or born elsewhere and immigrated at any time before 2060. That means deaths in 2060 depends upon unknown amounts of future immigration between 2023 and 2060. A 30 year old immigrant today still has a good chance of being alive in 2060 at the age of 67. (Immigrants often apply for family re-unification so some children might arrive later and they have a smaller chance of dying in 2060.)

Immigrants tend to be young, at the time they immigrate, so the “natural increase”, as defined, can be positive between 2023 and 2060 even if TFR remains 1.7 from 2017 to 2060.

A quote...

“The average age of newly arrived legal and illegal immigrants was 31 years in 2019”