JERUSALEM -- The world Jewish population is decreasing because of a low birth rate, assimilation and late marriages, according to a worldwide conference of researchers in Jewish demography, who predicted that by the year 2000, the world Jewish population will be little more than 12 million.

According to statistics presented at the Symposium on World Jewish Population, the world Jewish population declined from 16.6 million in 1900 to 12.8 million in 1985. The largest Jewish communities are found in the United States, Israel, the Soviet Union, Great Britain and France.

The 5.6 million Jews in the United States were reported to be undergoing changes found in most of world Jewry, including those in Third World countries, researchers said. The number of U.S. Jews has not grown since World War II.

"American Jews have the distinction of having smaller families than virtually every other ethnic group in the country," said Sidney Goldstein, a professor at Brown University and head of the Population Studies and Training Center.

"Jews are marrying later, expect to have smaller families and practice birth control more effectively than other any group," he added.

Goldstein said the birth rate of U.S. Jews is 1.6 per mother, about one-third the rate of the general population. He said researchers agree that the number of American Jews will not grow in the next two decades.

In addition, Goldstein said, a growing number of young Jews are delaying marriage. He cited a study by the American Jewish Committee, which polled persons who were college freshmen in 1971, and found that in 1980, 33 percent of the men and 40 percent of the women were married. These percentages, he said, were below the national average.

The trends among European Jews are similar to those in the United States. British Jews, for example, were reported to number 450,000 in 1951, and 330,000 today.

"We're automatically losing by natural decrease 1,500 people per year," said Marlena Schmool, a demographer for the Board of Deputies of British Jews.

Latin American Jews also have declined sharply. Argentina's Jews decreased fom 310,000 in 1960 to 230,000 today. The decreasing population of Jews in South America, researchers said, contrasts with the rapidly growing general population.

The researchers said the birth rate of Jews was in proportion to their level of Jewish observance.

In Antwerp, Belgium, the city's 15,000 Jews, most of whom belong to Orthodox synagogues, have registered an annual natural increase of 150, an exception among Jews in Europe.

Researchers said Israel remains the only country in which Jews are increasing. The birth rate for Jews is 2.8 per mother, and nearly 8 children per mother among the ultra-Orthodox.

Roberto Bachi, Israel's leading demographer and chairman of the conference, said Israel's 3.7 million Jews have experienced little intermarriage and have far more children than their co-religionists abroad.