In his fourth visit to Mexico in 20 years, Pope John Paul will encounter a country whose citizens -- however enthusiastic their welcome -- have moved away from church teachings in one crucial area. A remarkable drop in the birth rate reflects economic conditions, a more urban population, a vigorous state campaign -- and, quietly, in the most remote places, perhaps to representatives of the church itself.

CHIMALHUACAN, MEXICO CITY -- By defying Church teaching on the key issue of birth control, Mexico has changed profoundly from the country that first welcomed Pope John Paul II twenty years ago. Then, the average family counted seven children. Today, as the Pope makes his fourth visit to this overwhelmingly Catholic country, the average has fallen to 2.5 children, and planners aim to drop that number to about 2.1 children per family.

In this treeless, bustling community at the edge of sprawling Mexico City, already one of the world's most populous (18 million persons), it seems hard to believe you are in a country with a vigorous state population policy. Chimalhuacan grew from 60,000 inhabitants in 1980 to 412,000 in 1995, the last official count. Many came from the countryside because they found less land to go around near home villages. Beyond the reach of the city subway system, its streets a maze of mud each time rain falls, the place is nevertheless a magnet for the jobless dreaming of work in the capital.

It is also a place to see why the government policy works in a country where 9 out of 10 persons say they are Catholic.

"I respect my faith, but if I have more and more children, it means that to buy shoes for one means the others go barefoot," says 35 year old Maria Louisa Rosario, who has "only four" and has used an intrauterine device intermittently for sixteen years. Nearby stands her 16-year-old daughter, Gaby. "I tell her not to fill herself up with children," Rosario says quietly. At a government clinic, mother and daughter can receive birth control supplies from pills to sterilization.

Underlying economic pressures have helped assist government campaigns on radio and television, which have aimed, for instance, to convince people that "small families live better." "The success of the family planning programs comes from the effect of one economic crisis after another since 1982," says a doctor who works in the commuinity. "Both husband and wife must work now so both salaries might be just enough to live."

Since 1994 alone, the year Mexico joined the United States and Canada in the Free Trade Agreement (NAFTA), the peso has dropped in value from about 33 cents to ten cents. One out of three women now works outside the home.

"People submit themselves to this because of the cost of living," says Jose Manuel Villegas, 44, speaking of birth control devices displayed on the porch outside his busy shop. He notes that the price of even the government-subsidized milk for the poor and elderly rose 25 percent this month. "My brother has 15 children, but if you have more than three today, you hardly survive."

From the 10th floor, glass-walled office of Dr. Rodolfo Tuiran, Secretary General of Mexico's National Population Council, communities like Chimalhuacan are invisible in the brown-gray haze that lies over the city. Tuiran, a sociologist and economist, thinks rapid urbanization pushed the population growth rate down from 3.5 percent a quarter century ago to 1.88 percent today. More than 7 out of 10 Mexicans now live in cities of 15,000 or more. With city life comes "less stigma and fewer social sanctions" for those who choose not to have traditional large families.

Also contributing is the public health care system -- often with long lines and not always backed by sufficient supplies but now within reach of almost all of Mexico's 90 million inhabitants. For more than two decades schoolchildren have received sex education and increasingly, sometimes over the public objection of Catholic citizen's groups, classes examining -- even questioning -- traditional roles of men and women. One teacher's guide urges children to ask, what relation exists between the quality of life and the number of children a couple has?

"We try to avoid conflict with the Church," says Tuiran. Indeed, even the Madison-Avenue style ad campaigns are subtle, such as the one that insists, "Population themes are everyone's business." It makes a point, however, of implying children are an issue for couples to decide about among themselves.

But differences are inevitable. The Church, for instance, is unmovable in rejecting any but natural controls, such as the rhythm method, and objects to the very idea of population growth goals, maintaining these can lead to rights abuses. When the leader of the National Pro-Life Committee recently accused the government of encouraging forced sterilization, Tuiran quickly met with bishops. 

Even differences with the Church have their subtleties because doctrine can be softened in remote communities. "In the poorest states where we work, Guerrero and Chiapas, local priests will invite us to give pap tests to women and reproductive education talks to the communities, especially where government services do not reach," says a director of a non-government family planning organization with tens of thousands of clients. It is a model the bishops might nevertheless be wary of. By the year 2000, one half of all the world's Catholics will live in Latin America.

Meanwhile, a series of TV mini-spots is targeting the Mexican idea of super-masculine machismo. The current campaign from the National Population Council, insists "Planning the family is a man's business, too," and plays like a soap opera with ongoing discussion between two men. In the last episode, a few months from now Tuiran reveals, "The macho gets a vasectomy."