A major change has apparently begun in world deaths rates, ending decades of large increases in life expectancy in Third World countries, according to a study by the Overseas Development Council.

Death rates generally have been declining rapidly since World War Ii, with the greatest progress in Third World countries as they have improved water, sewers, medical treatment and made other advances. But now, according to davidson Gwatkin of the ODC, the decline in death rates has "begun to falter, to give way to a confused, diverse, ambiguous situation marked by unexpected slowdowns in the pace of health improvements. . . ."

The life expectancy in the developed world is now about 71 years, compared with about 53 years in the Third World countries. Life expectancy in the developed world has begun to level off and is increasing only at the rate of .16 of a year annually. In the Third World, the expected increase in rate was .58 of a year annually, but the reported rate instead is now only .4 of a year annually.

Progress in combating disease and malnutrition had moved rapidly after World War II, up to the early 1960s. Projections by the United Nations and others suggested the trend would continue as it had in Europe -- continually improving until the rate of improvement began to slow when life expectancy neared what is believed a natural ceiling between 73 and 80 years of age.

Instead the Third World life expectancy rate may be leveling off 20 years below that in developed nations.

In most countries, there is still some improvement in life expectancy even though the rate of progress is slowing. But in a handful of nations that Gwatkin surveyed, including Sri Lanka and Bangladesh, the death rate actually increased over a number of years.

Gwatkin believes the reasons for the unexpected failure to continue progress are that economic conditions have worsened and the medical aid given by clinics in the Third World has a limited power to improve life expectancy.

Though some major diseases such as smallpox have been eliminated, other commonplace killers -- infant, diarrhea, malnutrition and pneumonia -- still are not being controlled by the limited medical resources in Third World countries.