A major French study released today has discouraging news for women who delay childbirth--it documents a significant decline in the ability to get pregnant after the age of 30.

In the largest research project of its kind, the scientists found that difficulties in conceiving are "slight but significant" among women in their early 30s and "marked" after 35. They studied women undergoing artificial insemination whose husbands were known to be sterile.

The results are reported in the New England Journal of Medicine.

The new study has "important" implications because of the increasing number of career women who have children at later ages, two Yale researchers say. But if the declining ability to have children is as great as it indicates, they suggest that "new guidelines" for counseling women may have to be formulated and the current trend "reevaluated."

"Perhaps the third decade should be devoted to childbearing and the fourth to career development, rather than the converse, which is true for many women today," write Dr. Alan H. DeCherney and Gertrud S. Berkowitz in an accompanying editorial. They contend that this "might be the best way to ensure that women who desire to have both a family and a career are able to do so."

DeCherney added in an interview that this suggestion was meant to be "provocative," but that age was only "one small factor" in the overall decision about life style. He also emphasized that additional research is needed to confirm the "striking decline" in reproductive ability in women over 30.

A spokesman for the National Institutes of Health said that several studies are already under way in this country on the effects of delayed childbearing.

Although it has long been assumed that fertility diminishes with age, it has been difficult to document whether this represents a true biological change or a downswing in sexual activity. The degree to which male fecundity, or reproductive potential, plays a role has also complicated earlier studies.

The French study got around these difficulties by following over 2,000 women who had been artificially inseminated because their husbands were sterile. It was conducted at 11 centers under the auspices of France's Federation CECOS, a group for the study of human reproduction, and INSERM, the French national institute of health and medical research.

The team found that the success rate of impregnation after 12 cycles of insemination--roughly a year--was about 73 percent for women 25 and under and slightly higher for those 26 to 30. But among women 31 to 35, there was a drop to 61 percent, with the success rate down to 54 percent for those over 35.

This contradicts, the Yale commentators said, the "old maxim" that female fertility peaks at the age of 25 but remains high until 35. But they noted that medical researchers cannot yet explain why the reproductive decline starts in the early 30s.

DeCherney and Berkowitz suggest that it may be due to an increase in female diseases and disorders of the reproductive tract. Animal experiments also show that aging affects the regulation of the ovaries by the hypothalamus and pituitary glands.

National statistics show that 8 percent of first births in 1979 occurred in women over 30, up from 6.8 percent in 1960. Second and third births in this age group are also up.