The world's seventh billion person was "officially" born today - but according to some experts, the UN might have jumped the gun.

The US Census Bureau says the most likely date the world population will reach seven billion is between March and April next year.

Sergei Scherbov of the Vienna Institute of Demography said there was a 95% probability that the figure will be reached between January and July 2012.

"People have no idea how many of us are here," he told the BBC.

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Even the United Nations's own population estimates chief, Gerhard Heilig, admitted it was "nonsense" to suggest anyone could pinpoint where the seven billionth child will be born.

Heilig told the BBC the UN recognises its own figures come with a 1-2 per cent margin of error. Today's population could actually be 56 million higher or lower than seven billion.

"There is a window of uncertainty of at least six months before and six months after the 31 October for the world population to reach seven billion."

According to the UN's population website, no-one can determine the date with an error margin smaller than about 12 months, as even the best censuses have "inevitable inaccuracies".

The UN's reason for naming a symbolic date as seven-billion day is to draw attention to the speed of population growth, with less than 13 years having passed between the six-billion and seven-billion milestones.

- HERALD ONLINE