Late motherhood -- the tendency of many women to delay childbearing until their late 20s or early 30s -- could be a "stable feature of the population scene" for years to come, according to a study released yesterday by the Population Reference Bureau.

Over the past decade and more, authors Wendy Baldwin and Christine Winquist Nord said, there has been a remarkable change in childbearing patterns among American women compared with those of the baby-boom generation, when women tended to marry young and then have lots of children.

Today, women are marrying later; only about half are married by age 24, compared with three-quarters a generation ago, and they are waiting much longer to bear children.

In 1970, 19 percent of first births were to women 25 or older; by 1982 it was 36 percent. The median age of women giving birth for the first time rose from 21.8 years in 1960 to 23.2 in 1982.

The authors believe that the increase in later childbearing is rooted in major social changes and could turn out to be a long-term phenomenon.

Among the reasons for the increase are the massive entry of women into the labor force in the past generation; increased education for women, which gives them more job opportunities; the availability of contraceptives and the recent economic pinch.

Since many of these social conditions are expected to continue, the authors believe that the tendency toward later childbearing could remain a long-term feature of American life. The authors note that this would be a return to the patterns of the 1930s, which were reversed in the post-World War II era when people started marrying and having children earlier.

Reviewing statistics on health problems associated with delayed childbearing, the authors noted that, according to various studies, infertility rates are higher for older women (6 percent for the 20-to-24 age group, 9 percent for 25 to 29, 15 percent for 30 to 34, 30 percent for 35 to 39, and 64 percent for 40 to 44), that difficulties in delivery do not seem to be a major problem for women under 35, and that birth defects (many not serious) are 1 in 1,000 births for women under 35, 3 in 1,000 for women 35 to 39 and 16 in 1,000 for those 40 to 44.

They said new scientific discoveries may reduce the odds of problems with later childbirth.

Baldwin added in an interview that since most women do not delay past 35, and since the odds of avoiding health problems are pretty good until 35 or even 40, most women probably would not encounter difficulties.

Baldwin said that does not signal a new baby boom or massive upsurge in the birth rate. The total fertility rate (the number of births for a woman during her childbearing years of 15 to 44) reached 3.7 in 1957, then dropped; it has been at 1.8 for the past several years. Similarly, total births reached 4.3 million in 1957, then dropped to 3.2 million in the mid-1970s and seem to have leveled off at about 3.6 million.