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. And there would be profound consequences for societies with "more grandparents than grandchildren".
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.
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." Prof Murray said: "We will soon be transitioning to a point where societies are grappling with a declining population." Report author Prof Murray argues: "On current trends there will be very few children and lots of people over the age of 65 and that's very difficult to sustain global society. Think of all the profound social and economic consequences of a society structured like that with more grandparents than grandchildren. I think Japan is very aware of this, they're facing declining populations, but I don't think it's hit many countries in the West, because low fertility has been compensated with migration.
The fall in fertility rate 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
Without migration, countries will face ageing and shrinking populations. The report, part of the Global Burden of Diseases analysis, says affected countries will need to consider increasing immigration. "But at a globallevelthere is no migration solution." Prof Murray explains.