Jumuna, a toothless old woman of uncertain age but a seemingly limitless array of facial expressions, hitched her sari and plodded resolutely along a rice-paddy levee toward a hamlet of mud huts about a mile off the nearest dirt road.

Trailed by an entourage of excitedly chattering children, she shouted a Bengali greeting that emptied the huts of a dozen women who had been pounding rice in primitive stone husking machines, or crushing ferocious-looking green peppers with a wooden mallet.

As a dai, or regional midwife, in this impoverished central Bangladesh district, Jumuna commands as much respect as the village headmen, because her knowledge of birthing can spell the difference between life and death for a mother and her progeny.

But although she doesn't seem to understand it, Jumuna and thousands of peasant women like her possibly hold the key to the very survival of Bangladesh in the next century. They are the vanguard of a desperate effort to prevent Bangladesh's 90 million population from doubling in the next 28 years to a number of people that cannot possibly be kept alive by the available food resources.

By plodding each day mile after mile from hamlet to hamlet, Jumuna in the past two years has convinced more than 100 women to trek the 10 miles to a voluntary sterilization clinic in the town of Shipur for a 15-minute tubal ligation operation that permanently ends their childbearing.

For her efforts, she receives 50 taka ($2) a month and 10 taka (40 cents) for each woman she brings to the spartan but clean government clinic. The woman who undergoes the operation receives the equivalent of about $4 for food and transportation, plus a new sari to wear after the surgery is performed.

In addition to the thousands of dais who are contracted by the government and voluntary agencies as "motivators" in Bangladesh's 65,000 villages, 40,000 government field workers are employed to carry the sterilization and birth-control message across the country.

The results of their efforts have been impressive:

More than a million women have been sterilized since Bangaladesh gained independence in 1971; just one private agency, the Bangladesh Association for Voluntary Sterilization, alone performed 67,000 tubal ligations and vasectomies in its 25 clinics last year; sterilization is increasing 25 percent yearly; a social marketing project is currently selling 1 million cycles of birth control pills a year and nearly 67 million condoms; and surveys have shown that 95 percent of Bangladeshi women have heard of birth control and can name at least one form of contraception.

But the dimensions of the problem, according to government statistics, are staggering:

With only three-hundredths of 1 percent of the world's land, Bangladesh is the eighth most populous nation, and 3.78 million babies are being born a year, making a net annual population growth of 2.5 percent.

With 2.25 million people added to the population this year, a nearly bankrupt Bangladesh will require an additional 355,000 tons of food, 785,000 new jobs and 281,000 new homes. Although food production has increased from 9.4 million tons in 1961 to 14.7 million tons in 1981, the food available for consumption has dropped from 16.5 ounces per person per day to 15.5 ounces.

Perhaps most discouraging is the fact that to reach the western average of 2.1 children per family, 60 percent of Bangladeshi women will have to use modern contraceptive methods. At present, less than 11 percent do, and Bangladeshi women currently bear an average of six children.

Even if in 20 years the number of children per family were to decline miraculously to an average of two, the nation's population would still soar to 200 million by the middle of the next century, far more than its projected food resources can sustain.

John Naponik, a New Orleans gynecologist with the U.S. Agency for International Development (AID) family planning division in Dhaka, admits that his imagination reels when he contemplates the scope of the problem.

But while accompanying a visitor to government sterilization clinics and to remote villages in rural Bangladesh, he also said he is encouraged by hard evidence that indicates that while a family planning movement has hardly taken off in Bangladesh, a growing number of women want to control the size of their families.

"They aren't concerned about macroeconomics, or for the sake of Bangladesh population control. Their picture of the world goes two acres on either side of their thatched-roof hut. But they are looking for lunch for themselves and their children, and they are just plain tired, tired, tired," Naponik said.

In fact, studies show that most families surveyed regard four children as optimum, compared to the actual average of six, and that the problem of supply of family planning services is greater than that of demand, the AID official said. This despite the nearly $25 million, or about a third of total AID funding to Bangladesh, that goes for family planning. It is the biggest U.S.-backed population control program in the world.

"They want family planning. We know that, and it's understandable when you see how they live. Our problem is getting the information, the materials and the services to them," Naponik said.

A case in point seemed to be a woman named Falani who was in the government clinic in Shipur after undergoing sterilization, and who said in an interview that had she known more about the procedure, she would have volunteered sooner.

The wife of a laborer who makes 24 cents a day when he can find work, Falani said she has had seven pregnancies, and that four children survive. She did not know her age, but remembers having her first menstruation a year after she married.

Severely stunted in growth by malnutrition, Falani looks like a child of 11. She is under five feet tall, weighs barely 50 pounds, and has a childlike face that she tries to cover with a veil when embarrassed.

But she said that after trying to feed and care for a husband and four children, she feels old.

She first heard of sterilization about four years ago, she said, but she was afraid of the operation. She briefly tried birth control pills, but gave that up when she started becoming dizzy, she said. She also said she would not allow her husband to undergo a vasectomy because she feared "something would happen."

Finally, she said, she met a dai who explained in detail the tubal ligation procedure and introduced her to other women in the district who had been sterilized.

"I wish I had talked to her before," Falani said.

A dozen women interviewed at random in this dusty farming village--some of them sterilized and others using contraceptives--had similar stories.

One mother of four said her husband refused to let her take pills or use a diaphragm, but that they were using condoms and had not had children in two years. Another woman, with 11 children, said she was sterilized last year, and now thinks two or three children would be ideal. One mother of two said her unemployed husband told her to go to the sterilization clinic, and a woman with five children who underwent a tubal ligation two years ago said she would advise her children to have only two children.

Saladin Ahmed, a Bangladeshi physician who runs an Association for Voluntary Sterilization clinic in Dhaka, said that after 20 years in family planning work, he sometimes feels as if he were on an escalator going the wrong way. But he said the hundreds of women who stream to his clinic each month are a source of encouragement.

Overcoming ignorance and deeply ingrained social strictures, Ahmed said, is a significant part of the problem.

For example, he said, the concept of time is alien to many peasants, so that when a woman forgets to take her birth control pills two days running, she will often take three the third day.

Also, he noted, a compulsion to produce sons after pregnancies that produce daughters tends to defeat family planning. Compounding the problem are Moslem inheritance laws that dilute already meager farm plots exclusively among sons. This, in turn, exacerbates poverty as the size of the extended family grows.

Ahmed said he feels under pressure to meet the population control challenge, but not without caution.

He said that his and other sterilization clinics operate under strict government regulations, and are not permitted to perform sterilizations on women with fewer than two children. Also, he said, they must be able to demonstrate that the client voluntarily submitted to the operation without coercion or inducement, and understood that alternative contraceptive measures were available.

But as he looks to the future of population control in Bangladesh, Ahmed said, the challenge looms even larger.

Currently, nearly half the country's population is under 15 years of age, meaning that an ever growing number of women will enter their reproductive years, Ahmed noted.

"It will be an uphill battle all the time, but we cannot afford to do nothing," he said.