The United Nations has designated Oct. 31, 2011, as the "symbolic" date at which the world's population reaches 7 billion. But rather than choose one newborn to represent the demographic milestone, as it did for the six-billion mark in 1999 by tapping a Bosnian boy whose family has since struggled to stay healthy and afloat financially, the U.N. has decided to let a thousand flowers bloom. A spokesman for U.N. Secretary-General Ban Ki-moon recently encouraged "all countries to pick a symbolic baby 7 billion."

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Several countries are heeding the advice. There's Danica May Camacho of the Philippines, who was born in Manila two minutes before the stroke of symbolism (a.k.a. midnight) on Sunday and welcomed into the world with camera flashes, a scholarship for her education from local benefactors, and a cake from U.N. officials, according to the Guardian. But there are also babies from Bangladesh to Cambodia laying claim to the title. The child rights group Plan International has recognized Nargis, a girl born today in India's Uttar Pradesh state and pictured above with her mother at a village health center, as the seventh billion human in an effort to highlight female feticide and India's skewed sex ratio.

As the stories of today's celebrated newborns suggest, and as the U.N. itself has admitted, today's 7-billion hoopla is primarily a useful fiction -- a chance to focus the world's attention on rapid population growth and its myriad implications rather than a real attempt at pinpointing the precise time and place of the demographic milestone.

Forecasting population growth, after all, is an incredibly messy business. While data from the developing world is getting better, countries like Lebanon, Somalia, and Uzbekistan haven't conducted costly censuses in decades. And the data countries do provide is often controversial. As the BBC points out, China has well-organized censuses but may underreport births because of its one-child policy. Undocumented immigrants and migrant workers make things even more difficult. In a rapidly changing world, it's also difficult to estimate future fertility, mortality, and migration rates.

All this uncertainty explains why the U.S. Population Reference Bureau claims the global population surpassed 7 billion weeks ago; meanwhile, the U.S. Census Bureau and Austria-based International Institute for Applied Systems Analysis argue we won't hit that mark until this coming spring, or even as late as 2014.

So how did the U.N. land on Oct. 31, 2011? Five demographers with the U.N. Population Division spent ten months developing statistical models that estimated fertility (the average number of children born to a woman) in each country using data from censuses, surveys, and national registers that the department refreshes every five years. But, as the Population Division explains on its website, these models typically have a margin of error of 1 to 2 percent.

That means the U.N. estimate could be off this year by plus or minus 56 million, if one assumes an error of 2 percent in populous countries like China, India, and Indonesia. If one assumes a 1 percent error on a global level, the world could reach the 7-billion mark six months earlier or later than today. "No one can determine the exact date of a 7 billion world population with an error margin smaller than about 12 months because of the inevitable inaccuracies in all demographic statistics," the division notes.

In other words, if we really want to cover our bases, we may want to celebrate the 7 billion milestone again next year.