When I read the recent report that researchers at the University of California at Los Angeles had found a way to help women over 40 bear a child, my first reaction was: What's so new about that?

When my mother had me in 1930 at age 35, that was considered old; since then, a growing number of women regularly have had children in their late thirties and early forties. Even in earlier eras, the "change-of-life baby" was hardly uncommon. I know of one woman who bore three children, at ages 42, 43 and 45.

What, then, was new? In the earlier cases, biological limits were stretched but not overcome or altered. Nor did the earlier cases raise in so pointed a way the question of when an older mother -- or father, for that matter -- might be too old. That question must now be asked.

If a woman cannot produce eggs, it was once thought, that meant the end of her childbearing possibilities. The UCLA researchers, however, found that by using eggs donated by another woman and fertilizing them with a spouse's sperm, the resulting embryo could successfully be implanted in the infertile wife's womb. As Marcia Angell, deputy editor of the New England Journal of Medicine, noted in an accompanying editorial: "The limits on the childbearing years are now anyone's guess."

For many women this was good news, particularly those professional women who, for one reason or another, deferred childbearing. There is now one way at least to beat that "biological clock" that terrifies many still young, but ineluctably aging, women. There would, of course, be an obvious price to pay for going this new route: The child would not be her genetic child, only what we have come to call her gestational child. Even so, for many couples that is better than no child at all.

Just how far might we want to go in extending the years of childbearing? Is this necessarily a good development or, once again perhaps, one of those medical advances we might want to think twice about? I believe we should move only slowly and carefully past this new frontier. We might ask, for a start, what our present biological knowledge reveals about late parenthood. Is it bad that fertility peaks at age 25 and drops dramatically after age 35? Not necessarily.

Women who bear children after 35, but especially after 40, have a higher risk of dying, as do the children they bear. This is by no means a great risk, but when combined with the additional age-enhanced risk of bearing an abnormal child, it suggests that biology is telling us something: have children earlier rather than later. In case one thinks this applies to women only, not those aspiring older men -- not true. In men over 60, some physicians believe, there is a greater possibility of defective sperm.

What about the welfare of the children of older parents? Solid scientific knowledge is lacking, but common-sense observation suggests a few points. Older parents will have a somewhat diminished physical capacity for the rigors of raising young children. A 50-year-old mother or father is hardly likely to get up in the night, or romp in the grass, with quite the ease of a 20-year-old, though perhaps that is counterbalanced by greater emotional stability. A much older parent may be less of a companion or model for a child or teenager. The father of a friend of mine was 64 when he was born. At 77, he told me, his father was not a pertinent model of adulthood for a teenage son.

A still more subtle, little-explored issue turns on the most optimal coordination of life cycles. Our society already has serious problems with middle-aged people trying to cope with elderly parents. That can be difficult even when the children are financially secure and their own children out of the nest.

It would be much harder, even radically unfair, for a 20-year-old or a young parent to have to cope with such a burden. But people who have children in their forties, much less their fifties or sixties, risk imposing precisely that burden on their children.

None of these issues seems to me utterly decisive in ruling out older parenthood. But together, they add up to a powerful message: Walk slowly, and with extreme care, into this new territory.

Daniel Callahan, a medical ethicist, is director of the Hastings Center of Briarcliff Manor N.Y