Fewer babies could mean an "economic miracle" for sub-Saharan Africa, with gains of $500 billion (400 billion euros) a year over three decades for the region, the UN Population Fund said Tuesday.
The State of World Population report said a total of 59 nations were poised for a "demographic dividend" when the working-age population outnumbers the rest due to declining fertility rates.
The United Nations agency said these nations -- almost all in Africa -- could follow the example of East Asian economies like South Korea whose rise since the 1970s was helped by demographics.
"Recent shifts in the age structure towards younger populations present an unprecedented opportunity to catapult developing economies forward.
"The 'economic miracle' experienced by East Asian economies could become a reality for many of today's poorer countries," the report said.
It said there was also evidence to suggest that the demographic dividends could make the transition to more democratic forms of government "more likely".
The report found that the share of the youth population peaked in around 2010 in the world's least developed countries and "has begun declining", meaning that the working-age population in those countries will more than double by 2050.
In Nigeria, the most populous nation in Africa, the report said the demographic wave could "treble per capita income in a generation" as long as it was accompanied by the right policies and investments.
"The right investments are education, particularly girls' education. Girls must go to school, they must stay in school," the fund's executive director Babatunde Osotimehin told AFP in an interview.
"We also believe that health services, particularly reproductive health services, must be made available so women can make choices over their lives," he said.
But the Nigerian, a trained doctor, warned that health systems in countries in west Africa affected by Ebola had been "overwhelmed" and said that this could have a knock-on effect for the care of pregnant women and children.
"You actually might end up with more fatalities from that than you would from Ebola," he said.
- 'Unmet need' for contraception -
The report said governments should be ready to take advantage of the demographic dividend as there was only a "one-time opportunity" for rapid economic growth offered by this windfall.
"Without a solid economic and policy framework to back it up, the demographic dividend may not be fully realised," it said.
The countries named in the report also include some nations from continents other than Africa like Afghanistan, Iraq, Papua New Guinea and Yemen.
The report said that all the countries were on a path of demographic transition that begins with a lowering of infant mortality rates, which in turn encourages parents to have fewer children and invest more in their education and healthcare.
It noted that women in developing countries "generally have more children than they desire" and there was "an unmet need for modern contraception".
"Today there are more than 220 million women who want family planning and are not getting it," Osotimehin said.
The report homed in on the development of East Asia -- defined as China, Hong Kong, Japan, South Korea and Singapore -- where average annual income per capita more than quadrupled between 1965 and 1995.
It quoted research by David Bloom, a professor at Harvard University's School of Public Health, showing that the "demographic dividend" accounted for up to one third of that rise in income.
"We reckon that if all of Africa does this and in a timely fashion it will add about $500 billion to the GDP of Africa per year. Its' huge. It's about a third of what (GDP) is now," Osotimehin said.
Global fertility rates internationally have been dropping since the 1950s, from an average of six children per woman to about 2.5 today.
While the trend could be an economic boon for the developing world, the report warned that it was becoming a problem for mature economies.
"This demographic reality, tied to the ongoing shift in the balance of world population from younger to older people, creates risks," the report said.
In developed economies "smaller cohorts of young people may be tasked with paying more per person for the pensions and healthcare costs of larger older populations".