Once upon a time, a year ago, a woman named Donna Fasano sought high-tech help in conceiving a baby. By accident, her fertility doctor implanted in her womb not only Mr. and Mrs. Fasano's own fertilized embryo, but that of another couple as well. And this past December, Donna Fasano gave birth to two genetically unrelated babies, one white, one black -- "twins," ostensibly, but about as much alike as Arnold Schwarzenegger and Danny DeVito. Like all such stories from the medical frontiers of baby-making, this one sounds a lot like science fiction. But at its heart, it is something much older -- something much more like a fairy tale.

Such cases, in which one baby is inadvertently switched for another, are hardly common in real life, but they are cropping up more often now, for two reasons. In the first place, the brisk traffic in embryos, managed by a booming fertility industry, guarantees that mistakes will occasionally be made. In the second place, when mix-ups do occur -- whether in the fertility clinic or the hospital -- DNA testing is available to catch them. Witness the case of Paula Johnson, who obtained a DNA test for her ex-boyfriend in order to prove that he was the father of her 3-year-old daughter and who found in the course of it that neither the putative father nor she were biologically related to the girl. They had, in fact, brought the wrong baby home from the hospital; someone else had been raising their child.

These stories are fascinating not so much because of their baffling scientific permutations as because of their mythic quality. They tap into a deep, ancient anxiety that has long been a motif of legends and fairy tales. The changeling, for example, is the baby left behind by malevolent fairies to replace the original they've snatched for themselves. (In premodern Europe, midwives were suspected of witchcraft in part because they were thought capable of presiding over such secret switches.) The theme of mistaken identity -- of birthrights dispossessed and ultimately reclaimed, of one child living out a life destined for another -- underlies the narrative not only of Bible stories like Moses in the bulrushes and of fairy tales like "The Princess and the Pea," but of great and kitschy literary characters, from Little Lord Fauntleroy to Daniel Deronda. And, of course, Mark Twain's "The Prince and the Pauper" and "Puddn'head Wilson," in the latter of which the mother of a baby born into slavery exchanges him with the master's son. Stories like Donna Fasano's are our contemporary versions of such parables, which is one reason why some of us can't read enough about them.

The truth is, though, that if the stories are similar, their morals are not. The old fables are about the reversal or confusion of social categories -- the prince who trades identities with the pauper, the high-born girl baby raised in the humblest of circumstances. And they inevitably end with each child restored to his proper place, which is to say his proper place in the social hierarchy, and with the principle of genetic heritage -- here synonymous with rank -- again triumphant.

Our contemporary baby-switch stories are different. They aren't primarily about social confusion, but about emotional confusion. And more important, their denouements are different: genes -- our updated version of bloodlines -- do not automatically prevail. Paula Johnson, for example, wants to keep Callie Marie, the daughter she has been raising, not to trade her for the child to whom she gave birth. And the guardians of Johnson's biological daughter, Rebecca Chittum, want to keep her, too. (For a while, the two sides clashed over visitation rights, but an agreement has since been reached.) Three years of nurture clearly trumped nature, though they did not erase it. "I love Gavin too much to give him up," said a South African woman of her 18-month-old son when she recently learned he had been switched at birth with her true offspring. "Yet I feel this incredible bond with my own child."

The Fasanos, whose baby-switch saga is surely the strangest of them all, did relinquish custody of the infant who is not genetically related to them. But they did so reluctantly, under threat of a lawsuit from the genetic parents -- and only after securing assurances that both they and the baby's "twin" would have visitation rights. For three months, the Fasanos had dressed the two boys in matching outfits, settled them down to sleep in the same crib and thought of them, equally, as their own. And, indeed, if the case had gone to court, the Fasanos would almost certainly have won since the law in every state but California favors the rights of mothers who carried and gave birth to babies over the babies' genetic mothers. "We're giving him up because we love him," said Donna Fasano, who couldn't help still referring to her "two beautiful babies."

None of this would particularly surprise adoptive parents, who know from intimate experience that the force of love can overturn the claims of blood. It wouldn't even surprise biological parents who have asked themselves, What if somebody came to me and said there's been a mistake, that this isn't really my son or daughter? To most of us, the accumulation of moments with one particular child -- every snapshot of his face we hold in our heads -- would make it unthinkable to trade that child in for another. That feeling both resonates with the old legends of lost and swapped children and runs counter to them. Science hasn't fractured our fairy tales, it has only complicated them. And perhaps even made them more magical. In the very oddity of the new baby-switch scenarios, technology has brought love to the foreground.