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.
Which countries are affected?
More economically developed countries including most of Europe, the US, South Korea and Australia have lower fertility rates.
It does not mean the number of people living in these countries is falling, at least not yet as the size of a population is a mix of the fertility rate, death rate and migration.
It can also take a generation for changes in fertility rate to take hold.
But Prof Murray said: "We will soon be transitioning to a point where societies are grappling with a declining population."
Half the world's nations are still producing enough children to grow, but as more countries advance economically, more will have lower fertility rates.
Why is the fertility rate falling?
The fall in fertility rate is not down to sperm counts or any of the things that normally come to mind when thinking of fertility.
Instead it is being put down to three key factors:
- Fewer deaths in childhood meaning women have fewer babies
- Greater access to contraception
- More women in education and work
In many ways, falling fertility rates are a success story.
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