'There has been a remarkable global decline in the number of children women are having, say researchers.
Their report found fertility rate falls meant nearly half of countries were now facing a "baby bust" - meaning there are insufficient children to maintain their population size.
The researchers said the findings were a "huge surprise".
And there would be profound consequences for societies with "more grandparents than grandchildren".
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.
In the UK, the rate is 1.7, similar to most Western European countries.
Whenever a country's average fertility rate drops below approximately 2.1 then populations will eventually start to shrink (this "baby bust" figure is significantly higher in countries which have high rates of death in childhood).
At the start of the study, in 1950, there were zero nations in this position.
Prof Christopher Murray, the director of the Institute for Health Metrics and Evaluation at the University of Washington, told the BBC: "We've reached this watershed where half of countries have fertility rates below the replacement level, so if nothing happens the populations will decline in those countries.
"It's a remarkable transition.'
Read more: 'Remarkable' decline in fertility rates
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