The population of the world will be more than six billion by next year - double what it was just four decades ago, according to the United Nations.

The UN Fund for Population Action is celebrating World Population Day on Saturday by releasing statistics which show the huge population growth which has occurred in just 40 years.

It took millions of years for the world's population to reach three million, but just 40 years for that number to double. One billion of the six billion are under 25, leading to fears that the world's population will shoot up even more in the 21st century.

"They will largely determine the pace of population growth in the next century by their decisions on the size and spacing of their families," said Dr Nafis Sadik, executive director of the UNFPA.

Falling fertility

The UN predicts that there could be more than 10 billion people in the world by 2050, although family size and fertility has begun to fall in many countries in Europe, China and India.

African countries account for the highest fertility rates, with women in Nigeria, Congo, Uganda, Ethiopia and Somalia giving birth to an average of six children each.

Dr Sadik says ensuring young people have more choice over their fertility is the main way of bringing down the population.

"If women can choose, they will have fewer children than their mothers did," said Dr Sadik. "Families will be smaller and population growth slower. That is the lesson of the last 30 years."

Family planning

It is estimated that 50 million births end in abortion every year. This compares with 140 million live births. But babies are still being born at a rate of three every second.

In 1994, the Cairo Conference on Population and Development agreed that reproductive health was a human right. The 182 countries which attended pledged to raise $17bn by the year 2000 for population control programmes, such as family planning information and maternity health care.

But, although developing countries agreed to raise two thirds of the sum and had already raised 75% of it by 1995, the rich countries have still only come up with less than half their pledge of $5.7m.