Two studies released last week document declining birth rates throughout much of the developing world, but they warn that without more effective family-planning programs the population bomb still threatens to explode.

Reports by the United Nations and the Worldwatch Institute, a nonprofit research group, say that despite the falling rate of growth world population will double by the year 2025. The population growth will outpace production of food, fuel and other commodities, placing enormous stress on global resources, the studies say.

"Despite the falling birth rate, greater effort to encourage small families is essential if countries are to avoid the collapse of physical, economic and social resources," said Worldwatch Institute researcher Judith Jacobsen, author of the United Nations-funded report, "Promoting Population Stabilization."

"People assume that a falling birth rate means the pressure's off. But.... much more work remains to be done to forestall economic and social disaster," Jacobsen said in an interview.

A study by the U.N. Fund for Population Activities details demographic trends projected to produce a world population expected to stabilize at 10.2 billion by the close of the 21st century. World population is now estimated at 4.6 billion.

The projections assume gradually lower birth rates, but Jacobsen said immediate and sustained effort is required to achieve those rates. Also, she said, much of the most explosive growth will occur in nations least able to cope with it, including Pakistan, Bangladesh, India and most of Africa.

Some of the good news in the U.N. study doubled as bad news: rising average life expectancy and falling infant mortality rates contributed substantially to the expanding population of the less-developed countries.

Average life expectancy in the industrialized nations rose from 65.2 years in the early 1950s to 72 years in 1980, while the global average lifespan rose from 47 in 1955 to 57.5 in 1980.

Infant mortality in Third World nations fell from 164 per thousand live births to 100 per thousand between 1950 and 1980. During this period the rate in the developed countries fell from 56 per thousand to 19 per thousand. Infant mortality today remains highest in several African nations, where it exceeds by more than six times the rate in the industrialized world.