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sions for retired people," he says. "All the young people will be gone."

He has a masters in economics and is looking for a job. If he can't find one he'll move to the EU. Like many parts of Eastern Europe, his country has been hit with the double-whammy of low fertility and high emigration.

Adnan lives outside Sarajevo with his mum, Fatima,

0.81, Samir KC says. "So, how low will it go? This is the big question for us."

It is something more and more countries will have to grapple with.

While half of the next billion people will come from only eight countries - most of them in Africa - in most countries the fertility rate will be lower than 2.1 children per woman, the number necessary to sustain a population.

In Bosnia-Herzegovina, one of the most rapidly declining populations in the world, 23-year-old Adnan Mevic thinks about this a lot.

"There is going to be nobody left to pay for pen who has surreal memories of his birth.

"I realised something was unusual because doctors and nurses were gathering around but I couldn't tell what was happening," Fatima says. When Adnan arrived, the then UN Secretary General Kofi Annan was there to christen him the world's six-billionth baby. "I was so tired, I don't know how I felt," Fatima recalls, laughing.

Adnan and his mum flick through photo albums. In one a tiny boy sits in front of a giant cake, flanked by men in suits and military khakis. "While other kids were having birthday parties, I was just visited by politicians," Adnan says.

But there were perks. Being the six-billionth bab