Marcy Darnovsky is the associate executive director of Oakland's Center for Genetics and Society, so she's used to hearing weird tales from the cutting edge of genetics: rabbits that glow in the dark, or people seeking genes for immortality. Nevertheless, she was flummoxed when she picked up The New York Times and found an ad asking: "Do you want to choose the gender of your next baby?"

Sujatha Jesudason, her colleague, was equally surprised when a similar ad appeared on Hotmail. Soon, the pair began noticing what seemed like a sex-selection advertising blitz -- the come-ons were everywhere, from in-flight magazines to publications catering to the Bay Area's Indian community.

The ability of ordinary couples to choose the sex of their offspring is no longer science fiction, thanks largely to MicroSort, a division of the Virginia-based Genetics and IVF Institute and the name of its trademarked sperm-sorting technique, which offers a strong likelihood of selecting one sex over the other. Although still in clinical trials, the availability of gender selection for the masses has begun to permeate popular consciousness via the company's ads, which tout sperm sorting as a tool for "family balancing."

Family balancing? MicroSort's Web site (MicroSort.net) doesn't really explain why couples might need to "balance" their families, and the company's media department did not respond to interview requests.

Gender preference, however, is nothing new -- history is full of old wives' tales about how timing or sexual position can result in one sex or the other. But there's no evidence any of these are effective. "People have been trying to influence sex selection for millennia -- it's just maybe finally there are some effective ways of doing it," says Dr. Richard Chetkowski, a Berkeley physician who uses MicroSort.

With sperm sorting, lab technicians capitalize on the difference in weight between the X and Y chromosome to separate male sperm from female using a technique called flow cytometry. The woman can then be artificially inseminated with the sex-selected sperm. But the method isn't foolproof, since some male sperm get caught up among the females and vice versa. As of January 2004, according to MicroSort.net,there had been 419 births in the United States thanks to the company's technology, with a 91 percent success rate selecting for girls and 76 percent for boys. Sorting and insemination through a MicroSort clinic will run you $6,000 minimum, with no guarantee of conception -- although the company offers a package rate of $9,800 for up to four insemination attempts. The sorting procedure alone -- if an outside doc is doing the insemination -- comes to roughly $3,000, with fees.

In the past, sex selection was available only to couples undergoing in vitro fertilization -- the embryos can be genetically screened for gender prior to implantation in the womb -- while women in some cultures have chosen the disturbing alternative of aborting fetuses of the undesired sex.

At this early stage in the technology, many sperm-sorting customers have used it in combination with in vitro fertilization, which is typically employed when couples can't conceive naturally, or when they must screen for a hereditary disorder. But the cheaper and far less invasive insemination option promises to fling open the doors of fertility clinics to healthy middle-class couples desperate for that little boy or girl.

This development worries MicroSort critics like Darnovsky, who dismisses "family balancing" as a meaningless catchphrase engineered to sell gender selection like laundry detergent. "The whole concept of family balancing is a really bogus one," Darnovsky says. "What is an 'unbalanced' family?" Clever marketing, she concludes.

And the market could be huge. Fertility experts estimate that one in a hundred American couples uses high-tech fertility treatments. But sex selection could potentially be used by just about anyone. Here's a back-of-the-envelope calculation on what the sperm-sorting industry stands to make if the concept takes off: Multiply the minimum cost of the sorting procedure and associated fees (roughly $3,000) by the average number of tries needed to conceive (three), the US birthrate (four million per year), and the percentage of the population interested in trying gender selection (Darnovsky recommends a conservative 5 percent, although some polls show it's as high as 35 percent). The result: a $1.8 billion industry.

In Darnovsky's view, getting the public comfortable with choosing a trait as central as a child's sex opens up a Pandora's box: What will parents be able to choose next? Charis Thompson, a women's studies professor at Cal who has done research on the fertility industry, says the genius of the term "family balancing" lies in its ability to transform a social anxiety (desire for a boy or a girl) into a clinical diagnosis with a recommended medical solution. She compares it to recently developed ideas about body shape "proportionality" that have become accepted as rationales for plastic surgery. "So if you have a too-small bust for a ratio to your hips, it's now a 'bodily imbalance,'" she says. "It's a medical concept -- it's not that you're vain. That seems to me very similar to the rationale in 'family balancing. '"

So far, there are no studies on how, or whether, sex selection affects family health or childhood development. But that's no deterrent to those who think it should be an option. "It's something people want to do," says Dr. John Robertson, director of the American Society of Reproductive Medicine's ethics committee. "If you have three boys and one girl, you don't need a study to say that might make them feel better and contribute to a happy family."

UC Berkeley's Thompson is careful to note that although MicroSort may have invented the marketing concept, the market was there for the taking. In the Bay Area, where having kids is expensive and parents pour considerable financial resources into raising high achievers, small families are the norm. Couples only have so many chances to try for a son or daughter. "There is as much social innovation as technical innovation," she says.

Although marketing fertility services to the masses promises to be lucrative, local doctors who use MicroSort caution that fertility treatments can be time-consuming and costly, and really needn't be pursued by couples unless they have difficulty conceiving on their own, or need to screen for inherited genetic disorders. Some docs say they discourage clients from making the decision too lightly. "I don't want them to be disappointed when [they don't conceive] or think, 'Gee, I had a fifty-fifty chance at doing this anyway and I just paid $15,000 to do it,'"says Dr. Carolyn Givens of San Francisco's Pacific Fertility Center.

Despite these caveats, demand has been steady. Givens says she provides sex-selection services for about three dozen couples a year, and four other physicians in her group offer the same services. "We do not advertise it, but we do know that people find out about it and come to us, and in which case we don't turn them away," she says. "I'd rather see somebody do it this way than to terminate a pregnancy because they felt so strongly about having a particular gender."

While sex selection is a personal choice, critics of the technology are concerned about broader social ramifications if it does come into mainstream use. In some parts of the world, prenatal embryo screening and selective abortions have led to gender imbalances; the 2001 birth ratio in urban parts of India was 903 girls for every 1,000 boys, according to the country's census. And in parts of China, the Population Research Bureau reports, 100 girls are born for every 107 to 120 boys.

Supporters of sex selection say that this scenario is unlikely in the United States, where boys and girls are generally equally desired. But the critics still worry about how it will affect birth order. A study by researchers at Cleveland State University found that the vast majority of Americans who say they'd use sex selection would do it to ensure a firstborn son. In light of studies showing that eldest children have more leadership opportunities, what will it mean, Darnovsky asks, to have a "nation of little sisters"? And how does allowing parents to select for sex shape their expectations for their children? "Here come these people who spend $10,000 or $20,000 to have a child of one sex, and they have the wrong sex -- then what?" Darnovsky asks. "Or okay, you get the girl, and what if she's not a girl who wants to have her fingernails painted pink?"

And then there's the "slippery slope" argument -- the concern that, as science allows for it, parents will want to select for other characteristics as well. As Jesudason, Darnovsky's colleague, puts it: "The first time, you want a girl. The next time, you want one with faster muscles, or one that's prettier, or a boy that's smarter."

The disability rights community in particular has questioned how genetic screening could change what we consider a "normal" human body -- would it be right to screen for dwarfism? Doctors can already screen for chromosomal abnormalities, such as Down syndrome, that cause serious developmental defects, but what about disorders or diseases that hit later in life? Should we let people select against depression? Asthma? Allergies? If scientists find a genetic link for homosexuality -- should people screen for that?

"I really don't think we're ever going to see people coming to us wanting to do IVF so they can have a taller child or one of particular eye color," Givens says. But she does foresee a time when doctors will be able to screen against potential medical problems. "If we found there was a particular gene that told you 100 percent that if you live long enough you'll have pancreatic cancer in your lifetime, that would be worth screening out because it's such a lethal disease," the doctor says. "If we know there was a gene that was an 80 or 90 percent chance that you're going to get a disease, I'd want to stop it -- even if it's later in life. "

But fertility experts stress that selecting for physical or health characteristics if the technology becomes available is far more complicated than sex selection. Not all disorders have a single-gene cause, and some serious illnesses, such as cancer, can depend on environmental factors. Desirable traits such as athleticism, musical talent, or mathematical ability may rely on the interplay of many genes, in addition to nurture.

Dr. John Robertson of the American Society of Reproductive Medicine dismisses the slippery slope argument as too speculative. "Who knows if it will ever be possible to select on the basis of, say, intelligence?" he asks. "The mere possibility should not stop us from doing nonmedical selection for gender variety now."

A prenatal screen for intelligence? Perhaps not, but there's little doubt doctors will soon be able to scrutinize embryos for a far wider variety of traits than is currently possible. MicroSort's critics recognize that the cat is out of the bag. Their aim is to get society talking about the ethical issues it raises, and how tomorrow's genetic screening technologies should be applied, before they become a fait accompli -- an ad in The New York Times.