The total fertility rate – the average number of children in the Netherlands.
Meanwhile, the global population has tripled since 1950, from 2.6 billion people to 7.6 billion, the report says. An average of nearly 84 million people have been added to the Earth's population every year since 1985.
Other factors have been shown to predict falling fertility rates, including better infant survival rates and later marriage.
"Marriage is one of the biggest drivers of having children all over the world," said Kiarie, who is not an author on the new report.
'The world is really divided'
While total fertility rates fell across 195 countries and territories in the data, they were split roughly between the above and below, Murray said. "Replacement" describes the total fertility rate "at which population is no longer alive, assuming no migration.
For example, a woman in Cyprus had one child on average in 2017, while a woman in Niger had 7.1. This range is lower than 1950s, in which total fertility rates ranged from 1.7 live births in Andorra to 8.9 in Jordan.
"The world is really divided into two groups," Murray said. "In a generation, the issue is not about population growth, it's going to be about population decline or relaxing immigration policies."
In countries that want to boost fertility rates, the creation of financial incentives for families, including parental leave, has been shown to have a small effect, Murray said. Only 33 countries, largely in Europe, were falling in population between 2010 and 2017, according to the report.
"The country that is most concerned about this is China, where the number of workers is now starting to decline, and that has an immediate effect on economic growth potential," Murray said. "In a place like India – that is still a replacement, but that is just a dramatic change."
In just the past years, Kiarie said, parts of Africa and Asia have lowered fertility rates. The countries that have declined are the lower rates of contraception, where the introduction of family planning made a more significant impact, he added.
"There's a rapid progress, but I think in terms of … the areas that have the greatest need for family planning, it's still pretty in Africa," he said.
Lifestyle continues to kill millions
Lifespans have also gotten older on average since 1950, climbing from 48.1 to 70.5 years for men and women from 52.9 to 75.6 years, according to the study. However, the study authors say that just because women live longer does not mean they are living in better health.
When it comes to fertility rates, Kiarie said that the UN goals are the number of children they want, and that is what they choose. "What is key for that is to be there, in the women's hands."
What often gets lost in discussing fertility statistics and population numbers, Kiarie said, is the focus on individual people, their desires and how to empower them to achieve those goals.
"How can we ensure that people do what they think is right for themselves?" he asked.
CNN's Yemisi Adegoke contributed to this report.