With more and more people preferring a small family, the world fertility is expected to continue declining and the total population will likely see some sort of stability in 40 years, a senior UN population official predicted.

In a recent interview with Xinhua, UN Population Division Director Joseph Chamie said that over the last 50 years, the world has made progress in slowing the population growth, raising life expectancy, lowering mortality and improving quality of human life, among other things.

"Look back at the past 50 years, there have been successful achievements. I'm cautiously optimistic that things are improving," he said.

He noted that the last century, especially the second half, had seen a "revolutionary, unprecedented" demographic change around the world, with the total population more than triple, from 1.6 billion in 1900 to 6.1 billion in 2000.

The world population currently stands at an estimated 6.4 billion and is growing annually at 1.2 percent, much lower than the record high of 2 percent in the late 1960s, Chamie said.

The number of children per woman came down from an average of five in the 1950s to less than three currently, and the annual net increase of the population fell to 77 million in 2004 from the peak of 87 million recorded in 1987.

The annual population increase has been falling since the late 1980s and is expected to slide to some 29 million by 2050, when the globe will see some kind of population stabilization, he continued.

Slower population growth is mainly attributed to lower fertility. "Family size is getting smaller, everybody wants fewer children," he said.

According to Chamie, the world is experiencing a major demographic transition, from high birth rate and high death rate to low birth rate and low death rate. When the fertility decreases to the replacement level of 2, the birth rate and the death rate will be in harmony.

Currently, there are 60 countries whose fertility is at or below that level, he said, hailing this phenomenon as a major achievement in the past decade.

But the top UN demographer cautioned that the rapid population growth period "is not yet over" and it is expected to last 25 to 40 more years.

Six developing countries -- India, China, Pakistan, Nigeria, Indonesia and Bangladesh -- account for half of the annual increments of the world population, with India alone making up 21 percent. By region, South Asia and Africa are the two fastest-growing areas.

Chamie said the developing countries, particularly those in South Asia and Africa, should continue to take measures to contain population growth.

"We're going to see still more billions growing, especially in Africa and South Asia, and with there we have to continue providing family planning services to those men and women who want to have fewer children," he stressed. "Once we can achieve that, we can get some kind of stability around 9 billion people for the world."

As to population ageing, Chamie said it should be seen as a challenge rather than a problem. "This is good news. This is something that humanity has been trying to achieve for many centuries -- long life."

The challenge of population ageing could be met by extending working age, adjusting social policies regarding the elderly, bringing in more migrants, among other policy adjustments, he said.

"We have to adjust. These people living longer have to continue being productive -- productive ageing."