The United Nations estimates that on Monday 31 October the world's population will reach seven billion. But how accurate is this figure?

Not only is the world's population supposedly reaching seven billion today, the charity Plan International has anointed a girl born in India as the seven billionth.

In reality, things are much less clear.

The UN's population estimates chief, Gerhard Heilig, describes it as "nonsense" to suggest anyone could pinpoint where the seven billionth child will be born.

And he says the UN recognises that its own figures come with a 1-2% margin of error. Today's population could actually be 56 million higher or lower than seven billion, Mr Heilig says.

"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," he told the BBC.

You are always going to be essentially guessing… We will never have a true, definitive figure Professor Mike Murphy, London School of Economics

The UN Population Division website adds that no-one can determine the date with an error margin smaller than about 12 months, as even the best censuses have "inevitable inaccuracies".

"In fact, due to very poor demographic statistics in a significant number of developing countries the uncertainty may be even larger."

The UK census of 2001 illustrates the problem. The population had been thought to be about 60 million, while the census showed it was closer to 59 million.

"The British figures were revised by more than 1% in 2001 and that's in a highly developed country," says London School of Economics professor, Mike Murphy.

There are plenty of other countries, he says - Nigeria for example - where recording systems are far less accurate, and some countries that have not held censuses for decades.

One child

According to some experts, the UN has 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.

Births in China The UN estimates fertility (the average number of children born to a woman) using census data, surveys, vital registers compiled by national statistical offices - and, in countries where fertility statistics are controversial, other sources such as immunisation data. A large number of data sources are used to estimate fertility in China, including vaccination studies and school enrolment ratios. The UN says fertility estimates for China are "very controversial", in China itself as well as in the rest of the world.

Sergei Scherbov of the Vienna Institute of Demography, meanwhile, says there is a 95% probability that the figure will be reached between January and July 2012.

Earlier this year, he and two colleagues published a paper arguing that the seven billion figure was most likely to arrive in 2012 or 2013 - and was almost as likely to occur in 2014 as in 2011. But since then, he says, new figures have come in that make 2012 more probable.

"People have no idea how many of us are here," he says, modestly.

Mike Murphy agrees. "As you can never get a true figure to compare, you are always going to be essentially guessing… We will never have a true, definitive figure," he says.

Out for the count No census for more than 25 years: Afghanistan

Angola

Burma

DR Congo

Eritrea

Faeroe Islands

Iceland

Lebanon Census data not received by the UN Dutch Caribbean

Lebanon (last census 1932)

Somalia

Uzbekistan

Western Sahara Source: UN UN 2010 World Population and Housing Census programme

China illustrates one kind of problem. While it conducts well-organised censuses, the one-child rule means that births could be under-recorded.

Mr Scherbov believes the average number of children born to a woman is about 1.4, but the UN used a figure of 1.7 between 2000 and 2005 and 1.64 between 2005 and 2010. That means the UN could have overestimated the population of China by several million, he says. By the same token, Mr Scherbov might have underestimated it by this amount.

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.

"It can focus attention of people all around the world on global population challenges," says Mr Heilig.

Plan International's goal in picking a girl born in Uttar Pradesh as the seven billionth member of the world population is to draw attention to sex-selective abortion - the practice of aborting female foetuses in countries where male children are prized.

India is also the country with the highest predicted population growth for 2010-2015 - 135 million compared to 80 million in China.

The charity knows of course, just like the UN, that the number seven billion is approximate.

One of its campaigns is designed to ensure that more children are properly registered - as otherwise they have no proof of identity and "no legal existence" - and it cites estimates that 51 million newborns go unregistered every year.

Ultimately, does it matter if the seven billion figure is 1-2% out, or more?

Not really, says Mike Murphy.

"It's not in the end a figure that people use to make specific decisions. It may inform the context in which these decisions are made, but it's not a figure used in decision-making," he says.

The figure plays a part in the debate about the world's ability to grow enough food to feed its growing population. But if population figures are uncertain, Professor Murphy says, this is even more true of food production figures.