His early arrival on Tuesday, at 12.52am, meant he was born on the day the world's population was projected by the United Nations to tick over 8 billion.

While it is impossible to know who exactly was the 8 billionth person, anybody born on November 15, 2022 may have been the one to reach this milestone.

Mergen's parents, Anand Batjargal and Lkhagvasuren Erdenetsogt are happy to claim the milestone for their son, who was born at Canberra's Centenary Hospital for Women and Children.

"We have no idea about this eight billionth person ... we could have given birth on the 25th but he wanted to come today so it's a very unique moment to be here," Mr Batjargal said.

The world's population has grown rapidly over the past decade. The population only reached 7 billion in 2011. The growth has been due to the increase in human lifespan and high levels of fertility in some countries.

However, the rate of growth is expected to decline, especially as fertility rates decline across the globe.

ANU School of Demography Emeritus Professor Peter McDonald said the world's population growth has been driven by high birth rates in west and central Africa and south Asia.

"They are the only parts of the world growing in any substantial number," Professor McDonald said.

Mr Batjargal and Ms Erdenetsogt moved to Canberra in June from Mongolia to study at the Australian National University. Mergen is their first child.

"We both are planning to study. We were actually initially planning to study together and then we found out we were having a baby and my wife's plans changed a little bit. But we're still planning to study together while we are in Canberra," Mr Batjargal said.

"We like Canberra because it's very comfortable to live in, especially for a family with a new child."

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Mr Batjargal was supposed to have his final exam on Monday but didn't make it as Ms Erdenetsogt was already in labour. The exam has been postponed.

"I have one more final exam and then summer break so we'll have plenty of time to spend together," he said.

The United Nations has projected the population will grow above 10 billion in 2058 but Professor McDonald said he had doubts about this due to low birth rates, particularly in east Asia.

"I think that's probably too high. I don't think the population will quite get to that level because of the low birth rates," he said.

Australia's fertility rate is 1.7 babies per woman, the latest figures from the Australian Bureau of Statistics show. This is below the level to replace the population, which is 2 babies per woman, but migration drives Australia's population growth.

"In Australia, migration makes a big difference to the number of births not to the birth rate," Professor McDonald said.

"It's not that migrants have more births per woman but it is because migrants come in at a young age, before they've had their children and they add to the population that has babies very quickly.

"That more than offsets the fact our birth rate is below replacement."

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