For the first time, the world has reversed itself and has stopped growing so fast, the U.S. Census Bureau reported yesterday.

Since 1970 the world's population has gone up 1.9 percent annually, while in the previous 15 years it had been rising 2 percent each year, according to the bureau's study, which was funded by the Agency for International Development.

"We didn't expect a downturn in the rate of increase until the early 1980s, but it's happening a decade earlier and it's very significant," said Samuel Baum, the bureau's chief of international demographic statistics.

The difference of a tenth of a percentage point "is not a tremendous decline, but it's important because the numbers are so high and it represents a change in direction, which is more important," Baum added.

"The encouraging thing is that the direction has shifted in all regions of the world except Africa, and even there a number of countries - Tunisia, South Africa, Mauritius and Reunion - have had substantial declines," he said.(The latter two are islands in the Indian Ocean.)

Baum noted that more people are being added to the world and predicted that a growth rate of zero "will not be reached, at the earliest, until the years 2020 to 2025."

The world's population increased from 3.5 billion in mid-1967 to an estimated 4.3 billion in mid-1977, the report said. "At current [1976] vital rates, the world registers a net gain of over 200,000 persons each day."

Baum said two trends account for the reduced rate of growth: a rapid decline in fertility and a not-so-rapid decline in mortality. "People are being born at a slower rate and deaths are not going down as rapidly," he explained.

The census report does not project future world population but the World Bank has predicted it will reach 6 billion by 2000; the United Nations has predicted 6.3 billion by that year.

A study released last month by University of Chicago demographers Amy Ong Tsui and Donald J. Bogue projected a 5.8 billion total and said the world's population time bomb may have been defused by birth-control programs in some of the most populous countries.

"Only 10 years ago, doomsday prophesizing called for mass starvation, world chaos and possible world war by the year 2000," their study said. "If recent trends continue, the world population crisis appears resolvable."

The new census report seems to support that optimism.

At the U.N. Fund for Population Activities, a sopokesman told Associated Press that the new figures show "the downward trend is sufficiently impressive to remove any doubt that fertility is falling sharply."

This is the first time since Adam that there has been an overall across-the-board decline," said Tarzie Vittachi, fund information director.

But he added, according to AP, "No one should imagine that the population problem is solved. It is still a question of apportioning world resources with a growing number of people."

The census report noted growth rate declines in less-developed as well as developed nations. It cited Sri Lanka, where the 1966 growth rate was 2-3 percent and the 1976 rate was 1.5 percent. The Philippines, Thailand and the Republic of Korea each had a decline of 0.7 percent; Colombia had a decline of 0.6, South Africa, 0.4, and Turkey and China, 0.3.

The estimate for China was made by John Aird, chief of the bureau's foreign demographic analysis division, and was based primarily on provincial population figures that have been released since the Seotenber 1976 death of Chairman Mao.

China's population was estimated to be 982.5 million, 23 percent of the world total.

Asia, with 2.5 billion people, had 58 percent of the world's total in 1977, the report said. Because of the sheer size of its base population, its population growth accounted for two thirds of the world's increase.

The United States had a growth rate of 0.8 percent between 1976 and 1977, compared with 1.1 percent between 1966 and 1967. The United States had 5 percent of the world's population last year. North America, Europe and the Soviet Union, combined, had 23 percent, the report showed.

Africa's annual growth rate between 1975 and 1977 was 2.8 percent. it has been rising steadily since 1950, when it was 2.1 percent.

India's growth rate was 2.2 percent in 1976 and 1977, about the same as in the mid-1960s.