Mexico City — For Juana de Gadillo, wife of a poor farmer in the Mexican state of Tlaxcala, her fourth child was enough. ''How can I have more when we can hardly feed ourselves,'' she asks. Disobeying her church, she now takes contraceptives by injection in a town 10 miles from her village.
Gladys Mumbi, from the Tumutumu area of Kenya, wanted four children but had seven. She learned about family planning from a volunteer health worker who came to her village. Her husband, a laborer, agrees she should use it.
Slim Sri Lankan Karunatilke Gamage will be married in September. He and his wife want only two children. She intends to use the pill.
Durga Devi, who has four children, had to go against her husband's wishes to seek sterilization in a New Delhi slum five years ago. ''I waited until he left for work one morning and went to a local clinic,'' she said. ''Now he's happy and I've recommended 10 other women for sterilizations.''
These are just some of the individuals in the third world who are taking their own steps toward limiting the size of their families since the first World Conference on Population in Bucharest, Romania, in 1974.
A World Population Plan of Action adopted at Bucharest stressed that couples had the right to choose their own family size. While East-bloc and third-world countries resisted family planning then, almost all the third world has since come to adopt it as an essential part of development.
What ideas to spread the word on family planning are working best?
Experts agree that, with the world's population of 4.7 billion shooting up by a Mexico (around 80 million people) every year, the third world needs a steady flow of better ideas for the next 50 years or so at least.
* The most controversial approach is direct, unremitting government and social pressure, combined with a range of incentives, used in China. It has drawn strong criticism from anti-abortion groups in Washington. It arouses misgivings in other democratic countries which recognize China's need to grow more slowly but see the extreme methods used as oppressive and unusable elsewhere.
* Using local workers instead of strangers. In Madras, India, 60 newly trained slum women have persuaded 3,000 others to adopt family planning.
* Using local networks with an interest in keeping villages prosperous. In 12 poor, cotton-growing villages in the Punjab, the area's new milk cooperative has bought a jeep for a local doctor to spread the word on health - and family planning. He works with village women trained in rudimentary health care.
''Farmers take for granted 24-hour coop care for their livestock,'' an organizer says. ''They are just beginning to realize that they can improve the care for their own children.''
* Using public praise and recognition. Six hundred villagers from Java, Sumatra, and other parts of Indonesia were agog last July 11 to find themselves visiting Jakarta for the first time. They were even more overwhelmed to meet President Suharto in person.
Many had never been on an airplane before. Two years ago when the first group was brought in, some panicked on the tarmac and refused to board. Others drenched expensive hotel rooms as they tried to take baths in the village way by scooping water out and pouring it over themselves.
But the objective was clear: heaping praise, medals, and publicity on people, not for accepting family planning, but for staying with it for five years or more.
Another form of recognition: a card nailed to the doorway of a home where family planning has been used for five years or more. The first lady of Jakarta, the governor's wife, watched as a green card indicating five years' use of an IUD device was nailed to the doorway of Anthony Juanda and his wife, Jenny, in one slum area. They had three children, the youngest, aged five.
* Education through the news media. In Mexico City millions of women still tune in to highly charged television soap operas every day that hammer home the message that fewer children mean a happier, more productive life.
* Other forms of drama: In Bihar State in northern India the Tata Iron and Steel Works has produced, in Hindi and in English, a 45-minute musical play that opens with a clean hut and a slovenly one on stage. The neat home houses a family with only a few children. The dirty one contains six children. The death rate in the village rises. A wise man urges everyone to listen to the social workers.
Lines of men and women dance toward spotlighted booths offering sterilization operations. Music booms out and pro-family-planning banners unfurl. ''Corny,'' says family-planning physician Dr. Pramila David, ''but effective.''
* Getting education, health clinics, and family-planning services out where the people are. In Sri Lankan villages, volunteer workers organize into teams to talk to every single household.
On the island of Bali, every family in many villages has its family-planning habits recorded in a book and discussed at village meetings.
* Incentives. In China, one-child families receive ''glory certificates'' which give them access to more living space (where possible), extra rice rations , wage supplements ranging from $2 a month in cities to $25 a month in villages, and more.
''The problem has no single panacea and no easy answers,'' says Dr. Lessel David, population consultant with the Administrative Staff College in Hyderabad, India. ''Each country must work out what works best for its own conditions.''
In New York, the executive director of the United Nations Fund for Population Activities (UNFPA), Raphael M. Salas of the Philippines, agrees. ''Every culture needs its own approach,'' he says.
The United States is providing $240 million this year to help countries and private international organizations and the UNFPA with family-planning programs.
The US pressed hard in Bucharest for the third world to limit population growth as an essential part of its economic growth. Since then, almost every third-world country in Asia, and most elsewhere, have adopted family-planning programs. About $2 billion are now spent worldwide on family planning, almost half by developing countries themselves.
China, India, Sri Lanka, Singapore, Tunisia, Mexico, South Korea, Japan, Indonesia, Thailand, and other countries have all managed to lower their growth rates.
Methods have varied. India and Sri Lanka have relied heavily on sterilization. Indonesia has used the pill and the IUD as well as male contraceptives. Yet some general lessons do emerge:
* Ask villagers to walk to central clinics, and they will probably stay home, as they have done in parts of India, Pakistan, and Bangladesh. Decentralize and you will prosper: Kerala State in India has scored successes by building health clinics and schools out in the countryside, where the people are.
* Raise literacy and the status of women, and provide jobs for women and you will do well: Sri Lanka, Indonesia, and Kerala State are prime examples.
Mexico has realized the urgent need to reach its villages: In Mexico City, Dr. Manuel Urbina Fuentes, the government's family-planning chief, says that 26. 3 million Mexicans - one in every three - lives in a settlement numbering fewer than 2,500. Eleven million live in hamlets of 500 or less.
By 1988, Dr. Urbina wants a rural health worker, equipped with 13 basic medicines and supplies of contraceptives, in every settlement.
* Argue with local Muslim leaders and you may have a Pakistan (97 million people, more than six children per average family), growth rate almost 3 percent a year. Take the time to explain to Muslim leaders (better still, have a more liberal school of Muslim thought) and you may end up like Indonesia - birthrate down to 31 per 1,000 and an annual growth rate of 1.8 percent a year.
* A nation's leader needs to be committed to population policies. Where he or she is - India, Indonesia, Tunisia, Sri Lanka, Thailand - progress is marked. When he or she is not - Egypt so far, Kenya, Pakistan, areas of West Africa - progress is minimal.
The rules don't always apply. In Bangladesh (population: 98.4 million, due to jump to 145 million by the year 2000) political will exists but administrative and social pressures are so overwhelming that only 15 percent of couples practice family planning. The figure is 26 percent in India; almost 60 percent in Taiwan, South Korea, and Thailand; and now above 40 percent in Mexico.
Foreign aid plays a part, but much work is done by private international groups such as the Population Crisis Committee in Washington, one of about 40 such groups in the US which plan and help carry out family-planning programs abroad.
A paradox is also emerging, which Steven W. Sinding, AID population division chief, sees as ''sobering'':
A decade ago plenty of donor money was available for family planning, but political misgivings were widespread. Today, misgivings are fewer, especially in the third world, but money is also short.
''We now know that family-planning programs can work effectively,'' Mr. Sinding says. ''And we know that aid from the private sector is critically important for starting up programs and bringing in new ideas.''
* While aid assistance rose from $160.5 million in 1978 to $204 million in 1983, the 1983 figure measured in constant 1978 dollars was a decline - to $140. 4 million.
This is happening just as the London-based, US-supported World Fertility Survey 1972-84 found that 52 percent of women in Asia and 53 percent in Latin America wanted no more children. (In Africa the figure was only 16 percent.)
* A powerful and vocal anti-abortion lobby in Washington, part of President Reagan's far-right support, has used election-year political muscle to tighten restrictions on US aid and to downgrade family-planning priorities behind economic development based on private industry.
To population groups such as the Population Crisis Committee, the new White House policy is a ''disgrace'' to the traditional US commitment to family planning.
''In the long run, effective family planning reduces abortion rates - that's been proved in Mexico, Chile, and elsewhere,'' says a spokesman for the committee.
To the American Life lobby in Virginia, the new White House strategy is a ''triumph.'' The pro-life lobby says the US has no right to tell other nations how to act. But one effect of the policy statement was to tell other countries that if they include abortion activities in their family-planning programs, they will receive no US aid through private organizations.
Asked if this was not contradictory, Judie Brown, president of the American Life lobby, replied, ''It means that countries who want US dollars will have to comply with US policy. It's them who come cap in hand to us.''
Third-world spokesmen are concerned that the US, the champion of family planning, is changing course in midstream.
''I am upset,'' says a senior government official in Mexico City. ''You Americans have been pushing us for 10 years to increase family planning. Now are you coming to our capital to say we have it wrong?''
UN sources hope the statement is a political concession to the US far right on the eve of the Republican convention in Dallas. They claim that UN programs, and the US contribution to the UNFPA of $38 million this year, will be unaffected. ''We fund no abortion programs at all,'' says spokesman Ed Kerner.
Meanwhile, if the world's population growth rate is to be slowed, the basic need is more than providing contraceptives, or even more food. Good cropland is becoming scarcer and scarcer. The costs of chemical fertilizer are rising. The basic need is nothing less than changing the ways third-world individuals see themselves, their families, their lives, and their futures.
Awareness is growing that two, or even four, children per family allows more progress and health than six or eight children.
The Mexico City conference has had scores of recommendations before it on enhancing family life, new targets for lowering illness and death rates, finding ways to stop cities growing so fast, the status of women, and many more.
In Bucharest 10 years ago the debate was family planning or economic development. In Mexico City the debate is family planning and economic development.
What is new at Mexico City? Raphael Salas of the UNFPA says one novel element today is growing government intervention in individual lives, as in China. A senior US officials agrees: To him, balancing individual and state needs is the crucial family-planning question for the rest of the century.
Long-term economic growth must continue to provide jobs and incomes. World growth rates fell from 2 percent a year to 1.7 percent in the last decade, but much more needs to be done.
Even if the UNFPA goal of stabilizing population growth by the year 2100 is achieved, world population will still be 21/2 times as large as it is today, at 10.2 billion.
Adds Mr. Salas: ''Even if economic growth in developing countries were to reach unprecedented levels - 5 or 6 percent a year ... that would still leave more than 600 million people below the poverty line by the end of the century.''
Mr. Salas, who is widely credited with helping to make family planning a respectable, urgent issue around the world in the last decade, says he remains optimistic.
He keeps his eye on the long term:
''The need now is to put reason behind human desires,'' he says. The aim is to achieve individual security with equal opportunity to develop in unpolluted, nonde-graded environments. And the task, he says, has only just begun:
''All these efforts in population in the last 15 years are but a few short steps in a thousand-mile journey.''