Discussion of population ageing often involves references to birth rates. This post aims to clarify some important concepts related to birth demographics.

Here's map showing the natality rate in each country:

Click on the map to view it at full size at its source.

Some notes on the map:

• 'A' in the map legend means 'to' in Italian.
• 'Birth rate' here means natality, i.e. the ratio of live births in an area to the population of that area; expressed per 1000 population per year (Source)

For some sense of what this means, cross reference the above map with the following chart showing the decline of birth rate in Japan between 1950 and 1904:

This chart is found at http://www.stat.go.jp/English/data/handbook/c02cont.htm

2.1, the number often quoted in articles discussing population ageing, refers to the replacement fertility rate for industrialized countries with low child mortality. It is based on the idea of having 2 children to replace the parents, with 'one-tenth of a child extra to make up for the mortality of children who do not reach the age of 15' (Source). Fertility rate is the sum of the live birth rates by age for women aged 15 to 49; the hypothetical average number of children to be born to a woman over the course of her lifetime, assuming a consistent live birth rate by age in the future.