What demographers call the Total Fertility Rate is the average number of live births per woman over her lifetime. In the long run, a population is said to be stable if the TFR is at the replacement rate, which is a little above 2.3 for the world as a whole, and somewhat lower, at 2.1, for developed countries, reflecting their lower infant-mortality rates.
人类学中有一个总体生育率（Total Fertility Rate）的概念，指每个女性在其一生中的平均生育数量。从长期看，若总体生育率等于替换率，则称人口规模是稳定的。就全世界而言，替换率为略高于2.3，但某些地方（主要是发达国家，因为那里的新生儿死亡率更低）要低一些，为2.1。