WASHINGTON -- Call this Y6B: the year of 6 billion, a milestone the world's population is expected to reach this weekend.

The birth of the planet's 6 billionth inhabitant, projected by the U.S. Census Bureau, will mark another historic first: The world's population has doubled in less than 40 years.

Despite a gradual slowing of the overall rate of growth, the world population is increasing by 78 million people a year. That's the equivalent of adding a city nearly the size of San Francisco every three days, or the combined populations of France, Greece and Sweden every year, according to a coalition of environmental and population groups.

"It took all of human history for the world's population to reach 1 billion in 1804, but little more than 150 years to reach 3 billion in 1960. Now, not quite 40 years later, we are twice that number," said Amy Coen, president of Population Action International.

Even with a decelerating growth rate, the number of humans on the planet could double again to 12 billion by 2050 if the current growth rate continues, the coalition projects.

The impact will be sweeping, the coalition of population groups predict. "Every 20 minutes the world adds another 3,500 human lives but loses one or more entire species of animal or plant life -- at least 27,000 species per year," it warns.