Total fertility rate (TFR) is the average number of children each women would have in her life time, assuming that current age-specific birth rates remain constant throughout her child bearing years. In a population with equal amounts of in and out migration, a total fertility rate of 2.1or less can achieve zero population growth in about two generations.
The current total fertility rate world wide is 2.9. In the more developed countries (with 30 percent of the world population) the current TFR is 1.6. And in the less developed countries (70 percent) the current TFR is 3.3. (PRB) Zero population growth (2.1 or less) is commonly assumed by environmental scientists as a necessary but not sufficient condition for human sustainability.
(Directory) March 20, 2000