Twenty-four percent of babies born in the United States last year were delivered by cesarean section, more than four times the 1970 figure, and the cesarean birthrate has risen steadily despite nationwide recommendations that the operation be done less frequently, a consumer group reported yesterday.

The District of Columbia's cesarean section rate -- 30.1 percent of births -- was higher than that of 10 states for which rates were available, and Maryland's -- 26.6 percent -- was next, according to a report released yesterday by the Public Citizen Health Research Group, a consumer organization affiliated with Ralph Nader.

Cesarean section is the most frequent major operation in the United States, performed about 906,000 times in 1986. "It is the number one unnecessary surgery," said Dr. Sidney M. Wolfe, director of the Health Research Group. He said his organization estimated that the nationwide cesarean section rate should be 12 percent, with rates as high as 17 percent at hospitals caring for high-risk patients and as low as 7 percent in community hospitals with mostly low-risk patients.

Dr. Warren Pearse, executive director of the American College of Obstetricians and Gynecologists, agreed with the Health Research Group's assessment that the national rate should be cut in half, but he said that obstetricians' fear of malpractice suits contributes to the high cesarean section rate.

In a cesarean section, the surgeon cuts through the skin and muscle of the abdominal wall and through the wall of the uterus to remove the infant.

The chance of a woman dying during a cesarean section is two to four times greater than during a vaginal delivery, and the surgery involves a longer hospital stay, higher complication rates and a longer recovery time.

Last year, the average cost of an uncomplicated cesarean section was $4,270, compared with $2,560 for an uncomplicated vaginal delivery.

Despite the greater risk to the mother, Pearse and others said that obstetricians move rapidly to perform cesareans if labor is not progressing smoothly because they fear that they might be sued if there are subsequent problems with the baby.

The report found that repeat cesarean sections accounted for almost half of the increase in the operations between 1980 and 1985.

The increase came despite a 1980 recommendation by a National Institutes of Health panel that most women who had undergone one cesarean section should be allowed to try to deliver vaginally in a subsequent pregnancy.

A uterus scarred from a previous cesarean section has some chance of rupturing during the muscular contractions of labor, although recent techniques have greatly reduced this risk.

Pearse said the American College of Obstetricians and Gynecologists issued guidelines in 1982 and 1985 concurring with the NIH recommendation and estimating that between 50 percent and 80 percent of women who have had a cesarean section can deliver vaginally in a subsequent pregnancy. But he said that doctors do not always inform patients of this option and that "it's not always easy to sell it to women."

The Health Research Group's report said that fetal distress and failure of labor to progress are both frequently given as reasons for cesarean sections, and it charged that both are invoked too often, sometimes without adequate efforts to stimulate labor or to assess the baby's well-being before opting for surgery.

Failure of labor to progress is often ascribed to fetopelvic dis- proportion, a term meaning that the baby is too large to fit through the mother's pelvis.

But Wolfe said local and regional variations in cesarean section rates were much too great to be explained by varying rates of such complications among different patient populations.

"There is no reason to believe that there is anatomical variation in women or babies around the country or in a state like Maryland," he said.

The Health Research Group report includes a detailed analysis of cesarean section rates involving every hospital and every doctor in Maryland, which in 1983 had the highest known rate of any state. Wolfe said Maryland is one of about a dozen states that make such data publicly available, identifying doctors by code numbers.

Hospitals in the Maryland suburbs of Washington had a higher average rate -- 29.1 percent -- than Baltimore area hospitals, whose average rate was 25.9 percent, reflecting the influence of local variations in practice, Wolfe said. The average rate in D.C. hospitals was 30 percent.

Figures for individual District hospitals were not available, but the cesarean section rate at Columbia Hospital for Women also is about 30 percent, according to Dr. John Niles, an attending physician at Columbia and a member of the board of trustees.

Niles said that "maybe half" of physicians at Columbia suggest the option of attempting a vaginal delivery to patients who have had a previous cesarean section. "I don't believe that physicians are doing sections unnecessarily," he said. "You're trying to get the best outcome for the patient . . . . The liability climate is definitely a factor."

Kent and Queen Anne's Hospital in Chestertown had the highest cesarean section rate of Maryland hospitals in 1986, 42 percent. Garrett County Memorial Hospital in Oakland had the lowest, 12.4 percent. Most hospitals in the state had rates between 18 and 33 percent.

Cesarean section rates among Maryland doctors varied even more widely. Three physicians performed no cesarean sections during 1986, and six delivered 50 percent or more of their patients by cesarean section. The doctor with the highest rate, 54.17 percent, practiced at Shady Grove Hospital in Rockville.

Dr. Reed V. Tuckson, D.C. commissioner of public health, said the District's high rate of cesarean sections may reflect the fact that it is an urban area with a larger proportion of pregnancies among older women and among poor women with inadequate prenatal care and other medical problems.

The report recommended that laws be passed requiring hospitals to disclose cesarean section rates to patients and to require a second opinion for nonemergency cesarean sections. It said that insurers should pay doctors and hospitals equal amounts for vaginal and cesarean deliveries, to reduce financial incentives to operate.