Milestone will be reached tomorrow, say demographers It could be a baby born in New York, an illegal immigrant crossing the border from Mexico, or a Cuban migrant landing on a beach in Florida. Somewhere, the United States will gain its 300-millionth resident, probably tomorrow, confirming America's status as the world's third most-populated nation, behind China and India. According to the US Census Bureau, the milestone will be reached less than four decades after the number of people in the country passed 200 million, and only 91 years after the first 100 million. The main reason, demographers say, is America's enduring popularity as a destination for immigrants. On average, a new migrant enters the country every 31 seconds, a significant factor in the bureau's population clock - which also accounts for births and deaths - rising by one every 11 seconds. The Census Bureau goes so far as to predict the timing of the 300 millionth arrival - tomorrow at 11.46am, GMT (7.46pm in Hong Kong). 'I may have gone out on a limb with this, but I've put this 300 millionth American as a male baby born in Los Angeles county, a Mexican, born to an immigrant mother, and I've pinpointed October 17 as the day it will happen,' said Bill Frey, of the University of Michigan's Population Studies Centre. His prediction pulls together the latest trends in immigration, geography and birth patterns in the US, which is defying a downward global trend of population growth in industrialised nations. While immigrants accounted for only 5 per cent of America's 200 million residents in 1967, they make up an estimated 12 per cent today - the figure bolstered by between 12 million and 20 million Mexicans said to be living in the US illegally. 'Immigration accounts for 40 per cent of growth in any given year,' Professor Frey said. 'Its impact on this current growth and forecasted growth is huge because we're going to continue to have these immigrants coming in, and with this constant flow comes an increase in the pool of babies born to immigrant parents.' The most recent figures from the census bureau support demographers' theories. Last year, the country's minority population stood at 98 million, or 33 per cent. Hispanics were the fastest-growing group at 42.7 million, a 3.3 per cent rise on the previous year. Elsewhere in the industrialised world, falling birth rates have meant static population levels, but experts say the US will continue to buck the global trend and perhaps reach 400 million by 2043. 'The US is unique and has two things that keep the growth rate higher than its high-income neighbours, such as western Europe,' said Michael White, director of the Population Studies Centre at Brown University, Rhode Island. 'Immigration has fuelled the country for its entire history and will continue to be important. No matter how the immigration debate turns out, the US will always be admitting individuals. 'Second are birth rates and fertility rates, which are higher than other countries. As a general rule, population growth is cumulative and the number of people we add depends on how many are already around.' Environmental concerns have been raised over whether the population is growing too quickly. According to the Connecticut-based Centre for Environment and Population, each American uses three times more water than the world average and produces 2.3kg of rubbish every day, five times more than in the developing countries. But doomsday predictions, such as overpopulation leading to mass starvation and the break down of American society, have proved wide of the mark. 'We don't have a bomb that needs to be defused as we go through 300 million,' Professor White said. 'I think we'll get there without too many bumps.'