How big has the fall been?
The study, published in the Lancet, followed trends in every country from 1950 to 2017.
In 1950, women were having an average of 4.7 children in their lifetime. The fertility rate all but halved to 2.4 children per woman by last year.
But that masks huge variation between nations.
The fertility rate in Niger, west Africa, is 7.1, but in the Mediterranean island of Cyprus women are having one child, on average.