MEXICO CITY — Sandivel Lopez Rodriguez, her infant daughter at her side, sells gum at a subway in central Mexico City, unaware that she is an example both of the success of Mexico's population program and the challenge it faces from a population that is still rising steeply.
"I only want one more child, and my husband agrees," says the young Mrs. Lpez. "These days it's not possible to have more and provide them with everything they need."
With more families like the Lpezes making two children the norm, where a generation ago six children or more were common, Mexico's population growth rate is expected to continue its decline.
But because more than half of Mexico's official population of 91 million is under 20 years old, the total population will still continue to rise sharply.
That's a challenge for Mexico, which according to economists must create nearly 1 million jobs a year, and build millions of new houses, plus thousands of new schools and roads, while trying to protect an environment already severely degraded.
It's also a challenge for the United States, which has received millions of job-seeking Mexican migrants, but now shows signs of immigration fatigue.
Mexico's track record of failure to provide jobs and services for a growing population has migration experts warning that the forces pushing Mexicans to migrate north across the border are not likely to lighten soon.
According to figures just released by the Mexican government, Mexico's population grew by 1.8 percent in 1995, continuing a reduction in the growth rate that demographers, economists, environmentalists, and other observers highlight as a positive trend. That figure is substantially down from the 3.2 percent average growth rate in the 1970s.
"The reduced growth rate is a quantifiable success," says Miguel Cervera Flores, general director of statistics for the National Institute of Statistics, Geography, and Information, or INEGI, Mexico's census bureau. "The problem is that while that was being achieved, the population base was still growing."
Mexico can expect to double its population to around 180 million over the next 40 years - unless, says Mr. Cervera, an even lower growth rate is achieved. "We are already doing better than some countries, but then there are Latin American countries like Argentina or Uruguay that are at 1 percent," he adds. "That is a good goal for us."
To keep the growth rate falling, Mexico plans to step up a widely successful urban family-planning program in more remote and culturally resistant rural areas.
"Mexico is a country of change, and population growth is just one area where most Mexicans are willing to recognize this need to change," says Araceli Reyes Aregli, a physician having a quick lunch with with her fianc at Mexico City's Pino Suarez market. "[My fianc] speaks of some day having two children, but I think one is already a lot to think about."
Mexico City's population of 20 million has recently stabilized, according to INEGI. But towns of 2,500 people or fewer are the areas least amenable to family-planning efforts, experts say, with many women in such areas still producing six or more children.
That is why Mexico's family-planning efforts have shifted their target from urban to rural populations. Now Mexican television runs an ad, for example, where a farmer uses his trim cornfield to explain to his son how people are like crops, growing best when evenly spaced and limited to a number their caretaker can handle.
The Mexican government has now set a goal of having 7 of every 10 women of child-bearing years using some form of contraception by the end of the decade. What worries many Mexicans, however, is that even with a falling growth rate their country will not be able to meet an expanding population's needs.
"We're going to be millions more over the coming years, which means an even stronger effort so that there are jobs and education and health [care] for everyone," says Juan Jos Evangelista, Miss Reyes's fianc. "The reality is that accomplishing that for everyone is not going to be possible."
"The demographic figures are important, but the fact remains that an expanding youthful working population could in other circumstances be an advantage," says Gilberto Lpez y Rivas, a migration specialist at Mexico's National Institute of Anthropology and History.
"The larger problem is economic and political," he adds, "and the crisis Mexico is in, more than the growing population itself, points to a geometric growth in migration."
Much of the migration to the US from Mexico up to now has not been from cities but from low-density rural areas, says Mr. Lpez y Rivas.
But increasingly, members of Mexico's middle class will consider migration as economic conditions affect their well-being, he predicts.
Even a robust economic recovery would probably only increase migration pressures, some research suggests. "The economic crisis has actually detained migration, but once there is a recovery and more money circulating, that will facilitate the process," says Roberto Ham, a demographer at Colgio de la Frontera Norte in Tijuana.
Mr. Ham does not discount the progress Mexico has made in lowering its population growth rate. But he emphasizes that Mexico's population "pyramid" by age group features a bubble among the brackets starting at age 15.
"We may be producing fewer children," he says, "but it's that working-age population that is going to continue growing and making itself felt."