The UN says the world's population has hit eight billion, just 11 years after passing the seven-billionth milestone.  After a big surge in the middle of the 20th Century, population growth is already slowing down.  It could take 15 years to reach nine billion and the United Nations doesn't expect to reach 10 billion until 2080.

It's hard to calculate the number of people in the world accurately, and the UN admits its sums could be out by a year or two.

But 15 November is its best estimate for the eight-billionth line to be crossed.

The eight-billionth person on planet Earth was born in the city of Martuni in the Gegharkunik region of Armenia, Gegharkunik governor Karen Sargsyan wrote on social media.

"The newborn baby girl named Arpi is considered to be the eight-billionth person on the planet.  She is a daughter of Karen Zakarian and Ani Kocharyan, residents of the village of Tsovinar in the Gegharkunik region.  The baby girl was born weighing 2 kilograms 900 grams, 49 centimeters tall, absolutely healthy,” Sargsyan said.  

It is to be noted that in previous years, the United Nations has selected babies to represent the five, six and seven-billionth children.  

According to the BBC, Matej Gaspar from the suburbs of Zagreb, who was born in July 1987, was named the world's five-billionth baby.  

The 23-year-old Adnan Mevic from Bosnia-Herzegovina was named the world’s six-billionth baby.

The 11-yearold Sadia Sultana Oishee from outside Dhaka in Bangladesh was reportedly named of the world’s seven-billionth babies.  

After a big surge in the middle of the 20th century, population growth is already slowing down, the BBC notes.

It reportedly could take 15 years to reach nine billion and the United Nations says the global population will peak in the 2080s at 10.4 billion.

But the Institute of Health Metrics and Evaluation (IHME) at the University of Washington and the IIASA-Wittgenstein Center in Vienna believe it will happen sooner - between 2060 and 2070, at less than 10 billion.

But these are just projections.