The analysis predicts there will be about 10.3 billion people by the mid-2080s, up from 8.2 billion this year.
The number is then expected to fall to about 10.2 billion by the end of the century, a figure 6% lower than was expected a decade ago.
Li Junhua, the UN undersecretary general for economic and social affairs, said: “In some countries, the birthrate is now even lower than previously anticipated, and we are also seeing slightly faster declines in some high-fertility regions.
“The earlier and lower peak is a hopeful sign. This could mean reduced environmental pressures from human impacts due to lower aggregate consumption"