7 Billionth Person Born (Or Maybe More. Or Less. Who Knows?)

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With the birth of Danica May Camacho in Manila at two minutes before midnight, the United Nations Population Fund announced that the world's population had hit a new landmark: 7 billion people now fill the blue spinning globe we call home.

Or maybe not.

The U.S. Census Bureau comes to a very different conclusion, pegging the world's current population at 6,971,933,858 -- a difference of more than 28 million people. In other words, the U.S. Census Bureau guesses that the U.N. has overcounted by more than twice the current population of California. It argues that the world's population won't reach 7 billion until sometime in March of 2012.

Other estimates are even further off.

The International Institute for Applied Systems Analysis, an Austrian group that studies world population, argues that the world's 7 billionth person might not be born until July 2014.

The U.N.'s number has many scratching their heads, and asking how the agency counts people. Just how did the U.N. reach its conclusion?

The U.N. admits the number is only an estimate.

Amid the millions of births and deaths around the world each day -- and the poor demographic information currently being gathered -- it's impossible to pinpoint the arrival of the globe's 7 billionth occupant with any sort of accuracy.

"All demographic projections suffer from two kinds of potential errors," wrote Sergei Scherbov, director of demographic analysis at the Wittgenstein Centre for Demography and Global Human Capital, in a recent paper. He argues that uncertain projections and incorrect data about the current population make it a challenge to precisely pin down an exact number.

"The sizes of many populations today are not known with high accuracy, including the population billionaires China and India and many countries particularly in Sub-Saharan Africa," he pointed out.

Indeed, to reflect the inaccuracy in its guess, the U.N. chose Monday to mark the day with a string of festivities worldwide -- and not one, but a series of symbolic 7-billionth babies being born.

In Uttar Pradesh, India -- the most populous state in the world's second-most populous country -- officials said they would appoint seven girls born Monday to join Danica as symbols of the seven billion.

"It would be a fitting moment if the seven billionth baby is a girl born in rural India," said Dr. Madhu Gupta, an Uttar Pradesh gynecologist. "It would help in bringing the global focus back on girls, who are subject to inequality and bias."

Demographers say it took until 1804 for the world to reach its first billion people, and a century more until it hit 2 billion in 1927. The twentieth century, though, saw things begin to cascade: 3 billion in 1959; 4 billion in 1974; 5 billion in 1987; 6 billion in 1998.

Now the rate of change seems to be slowing, the U.N. estimates.

"The rate of increase appears to be slowing. But the large number of people now in their reproductive years, 3.7 billion, means world population will keep growing for several more decades," a recent agency report states.

The U.N. estimates the world's population will reach 8 billion by 2025 and 10 billion by 2083. But the numbers could vary widely, depending on everything from life expectancy to access to birth control to infant mortality rates.

Dr. Eric Tayag of the Philippines' Department of Health said the birth came with a warning.

"Seven billion is a number we should think about deeply," he said.

"We should really focus on the question of whether there will be food, clean water, shelter, education and a decent life for every child," he said. "If the answer is 'no,' it would be better for people to look at easing this population explosion."

The Associated Press contributed to this report.