The fertility rate of a country is the average number of children that women from that country will have throughout their reproductive years. From 1800 until 1865, Japan's fertility rate grew quite gradually, from 4.1 children per woman, to 4.8. From this point the fertility rate drops to 3.6 over the next ten years, as Japan became more industrialized. Towards the end of the nineteenth century, Japan's fertility rate grew again, and reached it's highest recorded point in the early 1920s, where it was 5.4 children per woman. Since this point it has been gradually decreasing until now, although it did experience slight increases after the Second World War, and in the early 1970s. In recent decades Japan's population has aged extensively, and today, Japan has the second oldest population and second highest life expectancy in the world (after Monaco). In contrast to this, Japan has a very low birth rate, and it's fertility rate is expected to fall below 1.4 children per woman in 2020.

Between 1939 and 1950, Japan's annual fertility rate fluctuated between 3.1 and 4.4 births per woman, before spiking to 4.6 in 1947. This was the highest of the Axis powers listed, as European figures were generally much lower at this point in history. The increase in fertility that followed the Second World War was part of the global baby boom of the mid-twentieth century.