Replacement of generations
Replacement of the generations of childbearing age by the generations being born.
The replacement of a generation of mothers is measured by comparing the number of women in a generation with the number of their daughters. Replacement is guaranteed if there are as many daughters as there are mothers.
Because of the sex ratio at birth (105 boys for 100 girls) and low infant mortality, replacement level fertility occurs in developed countries when mothers have around 2.1 children.