Key points from article :
Half of countries have insufficient children to maintain their population size.
Profound consequences for societies with "more grandparents than grandchildren".
Study followed trends in every country from 1950 to 2017.
Fertility rate has dropped from an average of 4.7 to 2.4 children per woman.
In the UK, the rate is 1.7, similar to most Western European countries.
China's rate 1.5 is and has moved away from its famous one child policy.
The study was published in The Lancet journal.
