The total fertility rate (TFR) is the average number of children a woman would have over her lifetime if current age-specific rates held steady. It is the single most important predictor of long-term population change.

Replacement level is the fertility rate needed for a population to exactly replace itself from one generation to the next, ignoring migration. Globally it is about 2.1 children per woman — slightly above 2 because some children do not survive to reproductive age.

When fertility stays above replacement, a population grows. When it stays below, the population eventually shrinks and ages. Today the global fertility rate is about 2.3, just above replacement, and falling fast.

More than half the world's population now lives in countries with fertility below replacement. South Korea (0.7), Singapore (0.9), and Spain (1.2) have some of the lowest rates ever recorded. No country has returned to replacement after falling far below it.

Low fertility does not mean immediate decline — there is "demographic momentum" from large young generations still having children. But within a few decades, sub-replacement fertility translates into shrinking workforces, aging populations, and fiscal pressure on pensions and healthcare.