Scientists have broken one of human reproduction's most imposing biological barriers, successfully inducing a group of post-menopausal women to carry and bear children.

Doctors at the University of Southern California used a variation on standard techniques of in vitro fertilization to enable four of seven women who had experienced ovarian failure -- or menopause -- to deliver healthy babies, according to a report in today's issue of the New England Journal of Medicine.

"Traditionally what has been talked about is preventing or treating infertility," said Arthur Caplan, a medical ethicist at the University of Minnesota. "This is the first step toward expanding the capacity for fertility. . . . We've never done that before in human history."

Because the women in the study were no longer capable of producing eggs, they were not the genetic parents of their offspring. Sperm from their husbands was combined in a test tube with eggs purchased from younger, fertile women, and the resulting embryo was implanted in the uterus of each woman for the duration of the pregnancy.

The scientists did not reverse menopause. Instead, they proved it can be sidestepped, that even though a woman stops producing eggs at menopause her uterus remains physiologically capable of carrying a fetus well beyond traditional child-bearing years.

That finding raises a second possibility. If scientists succeed in their current efforts to preserve or freeze unfertilized eggs, then today's findings would mean that a woman might be able to save eggs from her fertile years and have a child later in life.

Doctors stressed that this is not likely to happen soon, and fertility researchers cautioned that late pregnancies carry a variety of other physical risks and stresses that make them less advisable than traditional pregnancies. The procedures described in the USC study also are so expensive that they are beyond the means of most couples.

Nevertheless, at a time when technology has made it possible for babies to be conceived in test tubes and for women to bear children for other women and made it sometimes necessary for the courts to be called upon to declare parenthood, medical experts said the new findings provide confirmation that the traditional norms of child-bearing have been shattered.

"As a culture, we are facing some profound questions about the meaning of motherhood, the meaning of parenthood and the meaning of children," said Barbara Katz Rothman, a sociologist at City College of New York. "Technology is pushing us in some bizarre directions."

"This is not a procedure that will carry the same risks as normal pregnancy," said Christine Overall, a philosopher at Queen's University in Canada. "It seems to me that what we really need is not technological innovation that holds out the tantalizing but unreliable promise of conceiving into your 40s, but instead the social support to enable us to have our children at an earlier age if we choose to."

The USC team, led by Mark Sauer, began its study with seven women between the ages of 40 and 44, all of whom had experienced earlier-than-usual ovarian failure. Fertilized eggs were then obtained from three fertile donors, who were paid $1,500 each, and fertilized with sperm from each woman's husband.

The women were given heavy doses of hormones to mimic the changes that women normally undergo during ovulation and to ensure that the fertilized eggs could be implanted in the lining of their uteri, a step crucial for the successful beginning of a pregnancy.

The researchers began with 91 eggs, 40 of which were successfully fertilized. Nine of those were implanted in the women's uteri. The outcome of the experiment, by the standards of in vitro fertilization, was unusually successful: five of the women bore children, one of them stillborn, all by Caesarean section; one woman had twins; one woman had a miscarriage; and one woman failed to get pregnant.

Researchers cautioned that it was difficult to say, from the size of the study and other small studies like it conducted around the nation, whether these success rates could be repeated for all older women, particularly those who experience menopause not in their early 40s, like the women in the study group, but at the more typical ages of 48 to 52.

But the researchers said they found no evidence to suggest that the receptivity of a woman's uterus to an embryo would decline significantly from her early 40s to her mid-50s.

"I'm not sure age is a criterion that means too much at this stage," said Sauer, who teaches obstetrics and gynecology at the USC Medical School. "Older women in the past have been actively discouraged from {in vitro fertilization}. They should be encouraged."