There is a notable worldwide decrease in the number of children that women have, researchers say.
Their report concluded that the decline in the fertility rate meant that almost half of the countries were now faced with a "baby bust" – meaning that there are insufficient children to maintain their population size.
The researchers said the findings were a "huge surprise".
And there would be far-reaching consequences for societies with "more grandparents than grandchildren".
How big has the fall been?
The study, published in the Lancet, followed the trends in each country from 1950 to 2017.
In 1950, women had an average of 4.7 children in their lives. The fertility rate almost halved to 2.4 children per woman by last year.
But that masks a huge variation between nations.
The fertility rate in Niger, West Africa, is 7.1, but on the Mediterranean island of Cyprus women have one child on average.
In the UK the figure is 1.7, comparable with most Western European countries.
How high should the fertility rate be?
The total fertility rate is the average number of children that a woman gives birth during her life (it is different from the birth rate, ie the number of children per thousand people per year).
When the rate of a country falls below 2.1, the populations will eventually start to shrink (this figure for the baby bust is considerably higher in countries with high mortality rates in childhood).
At the beginning of the investigation, 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 have reached this turning point where half of the countries have fertility rates below the replacement level, so if nothing happens, the populations will decline in those countries.
"It is a remarkable transition.
"It is a surprise even for people like me, the idea that it is half of the countries in the world will be a huge surprise for people."
Which countries have been affected?
More economically developed countries, including most of Europe, the US, South Korea and Australia have lower birth rates.
It does not mean that the number of people living in these countries is decreasing, at least not yet because the size of a population is a combination of the fertility rate, mortality rate and migration.
It can also take a generation before changes in the fertility rate can be applied.
But Prof Murray said: "We will soon move to a point where societies struggle with a dwindling population."
Half of the nations in the world still produce enough children to grow, but as more countries progress economically, more countries will have lower fertility rates.
We prefer to give our daughter the best of everything & # 39;
Rachael Jacobs, 38, from Kent, had her first and only child seven years ago
I always focused on my career. When I was pregnant, I was still concentrating on my career.
I now know that we can survive with what we earn as a family and still go on holiday every year. If we had more than one child, we could not go on vacation.
We prefer to give our daughter the best of everything than having several children that we can feed and dress.
My partner and I are also thinking about the future. We want to be in a position where we can help her financially with university or housing. I do not want to say that she can not go to a party or make a new Christmas jumper.
Why does the birth rate decrease?
The decline in the fertility rate is not due to the number of sperm cells or something that normally comes to mind when you think of fertility.
Instead, it is put down to three important factors:
- Fewer deaths in childhood, which means that women have fewer babies
- More access to contraception
- More women in education and work
In many ways, declining birth rates are a success story.
What will be the impact?
Without migration, countries will have to deal with population aging and shrinking.
Dr. George Leeson, director of the Oxford Institute of Population Aging, says that this is not a bad thing, as long as society as a whole adapts to the enormous demographic change.
He told the BBC: "Demography affects every aspect of our lives, just look out your window at the people on the street, the houses, the traffic, the consumption, it is all driven by demography.
"Everything we plan to do is not only driven by the numbers in the population, but also by the age structure and that is changing, so we have no fundamental control over it."
He thinks that workplaces will have to change and even the idea of retiring at 68, the current maximum in the UK, will be untenable.
The report, part of the Global Burden of Diseases analysis, says that the affected countries should consider increasing immigration, which could cause its own problems, or introduce policies to encourage women to have more children, who often fail. .
Rapporteur Prof Murray argues: "Over the current trends there will be very few children and many people older than 65 and that is very difficult to support the worldwide society.
"Think of all the profound social and economic consequences of such a structured society with more grandparents than grandchildren.
"I think Japan is very aware of this, they are faced with declining populations, but I do not think it has been hit in many countries in the West, because low fertility has been offset by migration.
"There is no migration solution at global level," says Prof. Murray.
But although the change can challenge society, it can also have environmental benefits because of the impact of our species.
What about China?
China has seen enormous population growth since 1950, from about half a billion inhabitants to 1.4 billion.
But she too is faced with the challenge of fertility rates, which were only 1.5 years in 2017, and recently abandoned her famous policy for a child.
The reason that developed countries need a birth rate of 2.1 is because not all children survive until adulthood and babies are probably a bit more male than female.
But in China the report shows that for every 100 girls born there were 117 boys who "imply a very substantial sex-selective abortion and even the possibility of child murder for women".
This means that more children must be born to have a stable population.
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