Figuring out who your parents were used to be a pretty simple proposition. They were the people who kissed your forehead when you headed out to school and signed you up for Girl Scouts and barked at you for leaving your bike in the driveway.

Now, forget it.

In this age of high-tech fertilization and conception, deciphering parentage can be a complex computation for which there is often no reliable formula. Is the parent the person who, knowingly or unknowingly, contributed his or her genetic material? The person who carried the fetus in her womb? The person who pours the Cheerios in the morning and reads "Good Night, Moon" at bedtime?

The case of Susan Buchweitz in Santa Clara County points up the messy consequences of a 20th century legal system grappling with 21st century technology.

She was a 48-year-old interior decorator when she gave birth to a boy who grew from a donated egg fertilized by donated sperm. She had tried in her early 40s to have a child, according to news accounts, and after two years of counseling, decided time was running out. She chose the egg and sperm donors carefully through her doctor at Fertility Medical Associates in San Francisco. She saw herself and her baby as a family with all that the concept implies: relatives, traditions, values, unbreakable bonds and unconditional love.

But when her son was 10 months old, she found out the wrong embryo had been implanted in her womb. It was one of several frozen embryos belonging to a married couple who had an appointment for implantation the same day as Buchweitz. Buchweitz's baby, and the girl born to the married couple, were born 10 days apart.

The doctor knew of the mistake right after both women were implanted, but he covered it up until a whistle-blower alerted authorities. (He is about to lose his license, and he had to pay Buchweitz $1 million in damages.) When the husband of the married couple discovered he was the biological father of Buchweitz's son -- the wife used a donor egg -- he sued for full custody.

Last month, a judge mediated a settlement in which the biological father, who had temporary visitation rights for almost two years, gets to share custody of Buchweitz's son, who is now 3. Buchweitz, who did not return phone calls, must send her toddler to the man's home several days a week.

Think about this.

Because this man's sperm was accidentally implanted in a woman who is a complete stranger, he has the right under current law to step into her life and the life of her son. He can make decisions about how he will be raised and where he will sleep and what schools he will attend and how he will spend his Easters and Christmases.

Here's the kicker: Under California law, if the woman had been married at the time of implantation, her husband would have been the presumptive father, no matter what the DNA testing revealed, and the biological father would have no claim to the child. But as a single mother, Buchweitz didn't have full parental rights to decide who should and should not be part of her child's life.

So the courts give lots of weight to genetics but only if there is a single mother involved.

"If she's married, the court has a marital family to protect," says Janet Bowermaster, a professor specializing in family law at the California Western School of Law in San Diego. But it is not as concerned about protecting a family established by a mother and child.

David Magnus, co-director of Stanford's Center for Biomedical Ethics, has been following the Buchweitz case closely. Though what happened to her and her son is a horrible fluke, he says the permutations of parenthood that arise from fertility treatments demand new legal and social models.

"This has all become very complicated, yet we still have simple stories about the birds and the bees," Magnus said. "The simple stories in the law and in our thinking about families are not universally true any more. We need new stories, new ways of thinking about family arrangements for the future."

It's too simple to rely so heavily on genetics to determine who should be mothers and fathers. "We are such a gene-centered culture," Magnus said. "There is an assumption that we are our genes."

Don Kilmer, Buchweitz's attorney during the custody battle, agrees that courts give too much weight to biology and not enough to what's best for a child like Buchweitz's, who now has to live a divided life.

"Genetic material is cheap," Kilmer says. "But the judge had no choice in this case but to give the guy legal standing. We need legislation that takes up issues like these that lead to confusion for the child and trauma for the parents. ... There needs to be certainty and finality to some things, and one of those is establishing who your parents are (early on in life)."

I asked Kilmer if the weight this judge and the current laws give to genetics would have been the same if the circumstances had been reversed. What if, after the fertility clinic's mistake was revealed, the biological father wanted nothing to do with the baby? And what if Buchweitz, upon learning of her baby's paternity, decided she wanted child support? Would any judge have forced him to take responsibility for a child he had no intention of producing -- and certainly no intention of producing with a woman who is a complete stranger?

"I honestly don't know," Kilmer said. "It's an interesting question. No one has ever asked me that."

It seems illogical and unfair that a man can use genetic material to claim a child he wants but, under the same set of conditions, not be held responsible for a child he doesn't.

These are the murky, unfamiliar waters that judges today have to navigate. Britain, the country that produced the first in vitro baby, has a Fertilization and Embryology Authority created by Parliament to oversee the many complicated questions that must be answered as technology hurtles forward. The United States has no counterpart.

"There is no controlling body to reflect all the clinical, ethical and legal issues around fertility technology," says Susan Wolf, professor of law and medicine at the University of Minnesota. "So because everything has been all over the map state by state, judges get cases that defy any statutory scheme. There's been a good deal of despair about whether this country is capable of centralized reflection much less centralized legislation."

In the meantime, a mother relinquishes her baby several days a week to a man she doesn't know and, according to several people familiar with both parties, to whom she has almost nothing in common. I understand a man's sense of connection and responsibility to a baby that carries his genes. I understand a mother's desire to raise her child with stability and consistency, in one home, with one set of values.

We need laws as sophisticated and complex as the lives we live today, ones that recognize that families are defined by, and bound to each other, by much stronger stuff than double-helixes.