One-third of all married men and women in America trying not to have children have undergone sterilization, according to a new study of family planning practices around the world.

The report by Worldwatch Institute, a nonprofit research organization, called sterilization "the contraceptive phenomenon of the "70s." It added, however, that Worldwide abortion is now possibly the most common nonpermanent form of avoiding pregnancy.

"The number of couples using sterilization now exceeds the number of those using any other single preventive family planning measure," the report said. While a worldwide total of 4 million couples were protected from pregnancy by sterilization in 1950, 4 million sterilizations were performed in Europe alone in 1975, according to the studay.

Bruce Stokes, author of the report, said the sterilization figures for the United States were based on a 1973 National Survey of Family Growth by the National Center for Health Statisties of the Department of Health, Education and Welfare. Figures for sterilizations since then came from the New York-based Association for Voluntary Sterilization, he said, and were adjusted for death and aging.

He explained that the Alan Guttmacher Institute, research arm of Planned Parenthood Inc., estimated that there were 26.6 million married U.S. couples of childbearing age (between 15 and 44) in 1976, and that 8 million of those are either expecting a child, trying to have children or naturally unable to bear them.

Of the remaining 18.6 million couples not interested in having children, 6 million have chosen sterilization as their contraceptive method, Stokes said.

"It's astonishing and very exciting," he continued. "It probably means population growth in this country is going to slow much faster than people expect."

Throughout the world, an estimated 75 million couples have chosen sterilization out of the 500 million or so that don't want a pregnancy, the study said. The contraceptive pill is second in overall use at 55 million, with condoms third at 30 million and the intrauterine device (IUD) next, in use by 15 million women, according to the report.

All other methods, including ineffective means, are employed by 65 million couples worldwide, while between 30 million and 55 million abortions are performed every year, Stokes wrote, one for every three live births.

He cited a study by Christopher Tietze of the Population Council as indicating that "the diaphragm or the condom, backed up by legal aboration is the safest means of contraception for women."

The number of sterilizations probably will grow, Stokes said, because the surgical procedures involved for women have been greatly simplified in recent years. Illiterate village women in Bangaldesh have been taught how to do it, he reported, and female sterilizations are now more common than male vasectomies in the United States.

Still, the study continued, more than half the world's couple uninterested in having children use no contraceptives at all. They are primarily the poor, the young, the unmarried and the rural. "No research break-throughs on female contraception are imminent," Stokes wrote, "And no new male contraceptive is likely to emerge soon."

"There's no reason why the (worldwide) birthrate should not begin to fall using the methods we now have available," he added at a news conference last week.

Early family planning campaigns suffered fron reliance on only one birth control method or means of distribution and often ignored local taboos or social obstacles, the report said. A crash program in India pushing sterilization, often by force, resulted in a major backlash there, while the inundation of Pakistan with birth control pills and condoms in the early "70 had no real effect on the birthrate.

"Many observers question whether supply creates its own demand," the sutdy said.

"The old demographic transition theory, which suggested that birth rates fall after development has occurred, is now in question," the report went on. "Recent studies suggest that the connection is less direct, that fertility may fall without significant economic development, and that improvement in living standards may even encourage the formation of larger families. . .

"Throughout history, when couples have decided a small family was in their own interest, they have limited the number of their children despite all obstacles - albeit imperfectly and at considerable cost."

Where birth rates are falling, the report said, there are always government-backed family planning programs, and where the rate remains high the programs are weak or nonexistent. The most effective programs provide what the report called "a smorgasbord of services," offering a range of devices and medical backup along with some kind o fcommunity involvement.

He cited China as the nation with "the world's most ortanized and decentralized family planning program," if not the most democratic. Couples are told they should not have more than three children, and childbearing is planned at the community level for each couple, in accordance with production and income targets.

Although condoms are sold door-to-door by Japanese saleswomen, in village markets in India and in vending machines on the streets of Bologna, Italy, wouldwide spending for all methods of contraception was about $3 billion in 1971, the last year for which 1960s.

spent on abortions, the study said.