In more than a quarter of American couples of child-bearing age, the husband or wife has been sterilized.

Slightly more couples are choosing the pill as a birth-control method. But a growing number of sterilizations for medical reasons make sterilization for the first time the most common bar to conception among married people.

These findings were reported yesterday by the Princeton University Office of Population Research, whose head, Dr. Charles Westoff, said they were among the most "dramatic" and "extraordinary" changes in birth-control practice of recent times.

In a report to the government's National Center for Health Statistics, Westoff and research associate Elise Jones said that in 1975:

Some 6.8 million couples chose sterilization for birth control, nearly as many as the 7.1 million where the wife used birth-control pills. SOme 3 million husbands and 3.8 million wives were sterilized. Use of the pill, subject of adverse publicity in recent years, had dropped somewhat from earlier years.

Couples of child-bearing age, defined as those with the woman under 45, totaled 27 million. Among more than a million couples, one partner (usually the wife) was sterilized for medical rather than birth-control reasons.

Among couples who planned to have no more children at any time, 43 per cent chose sterilization (compared with 31.9 per cent in 1973 and 13.7 per cent in 1965). Twenty-four per cent used the pill, compared with 28.5 per cent in 1970, the high point of pill use for this group.

Most of these estimates were based on interviews with a national sample of 3.403 white women. In some cases, blacks were included in the statistics.

"The number of contraceptive sterilizations among blacks and whites is about the same," Westoff said. "But in blacks most of the sterilizations are in women."

Westoff and Jones collected no statistics on sterilized unmarried persons. An Association for Voluntary Sterilization spokeswoman in New York said, "We're getting an increasing number of inquiries about sterilization from single persons, so we believe there are quite a few."

While pill use was down slightly overall, its use among younger women who intend to have children was increasing - from 51 per cent in 1970 to 59 per cent in 1975.

The greatest drop in pill use was among women married 15 years or more. Use of the IUD, or intrauterine device, also was down in almost all the women except those married 20 years or more.

Though some drug firm reports have hinted at more use of the diaphragm, the Princeton dats showed a continuing decline in its use - from 5.7 per cent of all couples in 1970 to 3.9 per cent in 1975. However, women intending to have children were using it slightly more.

Use of condoms also continued to drop overall, though in couples married fewer than 10 years it had increased. It was being used by 7.5 per cent of all couples, compared with 6.4 per cent using the IUD, 2.6 using contraceptive foam and 2.2 the rhythm method.

Westoff and Jones said the number of couples choosing sterilization was growing partly because they wanted "relief" from continuous use of a chemical or a mechanical device. Also, they said, "couples are more willing to accept irreversibility" in their decision not to have children.

The most common female sterilization operation - tubal ligation, or tying the fallopian tubes - cannot be reversed. The male operation - vasectomy, or surgical removal of the tube that carries sperm from the testicles - is seldom reversible.