After months of gradually stronger birth control warnings, Chinese authorities have begun to use intense social and bureaucratic pressure to ensure that every Chinese baby born the rest of this century is an only child.

Reports from foreigners and Chinese in both rural and urban areas say neighbors have been enlisted to visit nightly the homes of recalcitrant couples until they agree to sign a "one family, one child" pledge.

The government has assigned extra officials in several communes to carry out the pledge campaign, designed to cut the average rate of 2.3 children per family, and administer the penalties of play and benefit cuts to those who break the pledge.

In large cities such as Peking, where birth control programs have had more success, local supervisors have also made it clear that pledges must be honored. "We are not allowed to have two children any longer," said one worker here. The official People's Day said 5 million couples of childbearing age, about 29 percent of those who have one child, have signed the pledge so far.

The campaign frustrates the deep desires of Chinese peasants for enough children to care for them in their old age and carry on the family name. But an extraordinary set of official population statistics just released shows how desperate Peking leaders feel the situation had become: the 970 million population is likely to increase to 1.2 billion in the next 20 years, even with an energetic birth control program.

Since early 1979, when Peking began to call for strong birth control measures, the plan appeared to be a wholly voluntary system based on economic rewards for those who had one or at most two children, and penalties for those who had more. Recent reports indicate, however, that the government, perhaps because of its recent reappraisal of its severe financial difficulties, has decided to say less about birth control bonuses and rely instead on intense social pressure.

A foreign researcher who has been living for the last month in a commune headquarters in southern Hebei Province said, "People are going and knocking on doors every night and arguing with people who don't take the pledge." Commune officials assigned to pursue the campaign are told "you must not be afraid of being beaten."

A commune official said that 92 percent of the couples in that area with one or child or less had taken the "one-family, one-child" pledge. "But what are you going to do about those who don't sign?" the researcher asked.

"Everybody is going to sign the pledge," the official said.

In a Feb. 11 editorial at the top of the front page of the People's Daily, the Chinese people were told that "the immediate task is to shift the focus of family planning to advocating one child for each couple." The editorial indicated that the population was now about 970 million, but that because of baby booms in 1963 and in the late 1960s, huge numbers of young people would be marrying and having children in the next two decades, making severe restrictions on their family size necessary.

"At present young people and children under 21 make up half of our total population," the editorial said. "Before the end of this century, an average of 20 million of them will marry and bear children each year . . . Therefore we must from now on spare no efforts to advocate one child per couple."

The editorial said, "In dealing with the obstacles arising from force of habit, we should not adopt simple administrative means and coercive measures," an indication that some localities are indeed doing whatever they can to meet strict birth control quotas handed down from Peking. In Guangzhou (Canton), travelers report, women checking into hospitals to have their first child are offered large bonuses for postnatal sterilizations and being told they must have them after a second child. Officials have complained that in Henan Province many men are hiding to avoid required sterilization operation. Significantly, the People's Daily editorial made no reference to the economic and educational incentives for small families that have been promised in some areas, and indication that the government may now consider incentives too expensive and too disruptive of other programs.

The editorial says that birth control can be encouraged by wiping out ancient preferences for male babies, providing better old-age benefits, producing efficient contraceptives and publicizing the advantages of limited births for the national economy.

"We hope that, proceeding from the overall needs of achieving the four modernizations, married couples of child-bearing age would consider having one child a great honor," the editorial said. The "four modernizations" refers to China's plans to promote more rapid economic development.

Some provinces recently have promised money bonuses and preferences in school placements for only children but there is little evidence yet that such programs are widely in effect. An office worker here said promises by some organizations of more apartment space for one-child families were meaningless, since most of the available apartments are about the same size.

Economic punishments for an unauthorized second or third child also have been threatened, including salary cuts and reduction of ration coupons for the extra children.

The official New China News Agency recently released a set of population projections by a Chinese computer expert, a social scientist and two engineers. If every woman of childbearing age had three children beginning this year, the population would be 1.4 billion in the year 2000 and 4.3 billion by 2080. If the birth rate dropped to one child per couple by 1985, the population would rise to 1.054 billion by 2004.

The People's Daily indicated that the government has a more modest expectation -- something less than two children per couple, with the population peaking 1.2 billion in 2000.

China says its current population growth rate is 1.2 percent, down from a high of nearly 3.35 percent in 1963.

The agency article said, "Some comrades are worried" that cutting births sharply eventually will make China a nation of old people. But the scientists say it will be several decades before China, now with 4.8 percent of its people over 65, even approaches the more than 10 percent elderly rate in the United States and other Western countries.

The article also scoffed at worries that China will suffer a labor shortage. Even with one child per couple, the labor force will jump from 520 million now to 760 million in the year 2000, the article said.