Almost half of the world's nations, including Ireland, are experiencing a 'baby bust' because there are not enough children to maintain their population size.
A study, published in the Lancet, followed fertility trends in every country from 1950 to 2017 and found that 91 nations are not producing enough children to maintain their current populations.
It measured the total fertility rate – the total number of children born or likely to be born to a woman during her lifetime according to the prevailing rate of age-specific fertility in the population.
A value of 2.1 is generally taken to be the level at which a generation would replace itself in the long run, ignoring migration.
In Ireland, the fertility rate dropped below the replacement level in 1989 and again in 1999 and has remained there since. It fell by more than 26% between 1986 and 2016 - from 2.44 to 1.81.
In 1950 women were having an average of 4.7 children in their lifetime. The fertility rate almost halved to 2.4 children per woman last year.
Ireland's fertility rate, at 1.8 was similar to rates in England and Northern Ireland. The rate in Wales was 1.7 and in Scotland, it was 1.5.
The fertility rate in France was 1.8; it was 1.4 in Germany and Spain and 1.3 in Italy.
The lowest fertility rate was in Cyprus where, on average, a woman would give birth to one child throughout her life, as opposed to the highest in Niger, where a woman would give birth to seven children.
In addition to Niger, Mali, Chad and South Sudan were among the 104 nations with fertility rates exceeding two births per woman.
However, 91 countries, including Singapore, Portugal, Norway and South Korea, along with Cyprus have rates lower than two.
Director of the Institute for Health Metrics and Evaluation at the University of Washington, Dr Christopher Murray, said the statistics represented a “baby boon” for some countries and a “baby bust” for others.
“The lower rates of women's fertility clearly reflect not only access to and availability of reproductive health services, but also many women choosing to delay or forgo giving birth, as well as having more opportunities for education and employment,” he said.
The findings are included in the annual Global Burden of Disease study, to which 3,676 collaborators from 146 countries and territories contribute.
The global population almost tripled since 1950, from 2.6 billion to 7.6 billion last year.
“Although total fertility rates are decreasing the global population continues to grow as death rates decline and because of population 'momentum' in previous decades,” said Dr Murray.