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Better Than SexThe growing practice of embryo eugenics.

By William Saletan
Posted Saturday, Sept. 16, 2006, at 7:52 AM ET

"Mommy, where did I come from?"

Throughout history, parents have squirmed at that question because it involved sex. Now, many are squirming because it doesn't. For children born through in vitro fertilization—3 million and counting—the answer involves injections, selections, and lab dishes. The hard part is explaining the siblings we rejected: nearly half a million embryos frozen in U.S. clinics alone. For thousands of children, the story now includes preimplantation genetic diagnosis, a technique for weeding out flawed embryos.

What flaws are we screening for? That's the most uncomfortable question of all. Sometimes the flaw is a horrible disease. But increasingly, it's a milder disease, the absence of useful tissue, or just the wrong sex. If you think it's hard to explain where babies come from, try explaining where baby-making is going.

In its early days, PGD targeted fatal childhood diseases such as Tay-Sachs. But a new survey of U.S. fertility clinics, scheduled for release this week by the Genetics and Public Policy Center, suggests the line is moving. Among clinics that offer PGD, 28 percent have used it to target genes whose associated diseases don't strike until adulthood. The list includes Alzheimer's, which afflicts some people in their 30s but usually arrives much later. According to next month's Journal of Clinical Oncology, PGD has also been used to wipe out colon cancers that don't develop until age 45 to 55 and are treatable, if detected early, with survival rates of 90 percent.

For some of these adult-onset genes, the risk of illness is less than 50 percent. But it feels mean, even arbitrary, to quibble about probabilities. American clinics target these genes anyway, to prevent "cancer predisposition syndromes," if not cancer itself. Even if your child never gets sick, just knowing he has the gene can cause anxiety, as British regulators noted four months ago when they approved PGD for colon cancer.

Probability and life span aren't the only standards we're relaxing. We're also applying PGD to less serious diseases. Encouraged by Britain's ruling on colon cancer, a London hospital is proposing to prevent autism by eliminating male embryos, which are more likely than females to get the disease. Two weeks ago, the New York Times described an American patient who plans to screen her embryos for an arthritis gene. The probability that the gene will cause the disease is only 20 percent, and if it does, the disease is highly manageable.

Once you screen for one gene, it's tempting to screen for others. The woman who's targeting arthritis, for example, added that gene to an already-planned test. Another patient, described in the same article, set out to scan his embryos for colon cancer and ended up chucking two more for Down syndrome. "You kind of feel like you shouldn't be doing it," his wife confessed. "But then why would we go through all of this and not take those extra precautions?" Soon, you're hunting even for dormant genes. A PGD technique unveiled three months ago can find genes that won't harm your child but might, if combined with other genes, cause disease in a later generation. British patients are already asking clinics to filter out embryos carrying such genes.

If you're screening embryos for the benefit of future kids, why not do it for kids already here? Some embryos are particularly suited to donate cord blood to a sick elder sibling; nearly one-fourth of American PGD clinics have sorted embryos for this purpose. Originally, this was done for diseases such as Fanconi's anemia, in which the embryo could be checked both for its own sake—to avoid the bad gene—and for the blood match. Today, however, it's also being done for leukemias that have no clear genetic cause. In the GPPC survey, 6 percent of clinics that perform PGD admit they've used it to identify which embryos are useful donors, even when the test offers the embryos no benefit.

Once you start treating a child as a tissue bank, it's hard to stop. Last month, after a Swiss couple used PGD to pick a donor embryo for their ailing son, a British clinic director scoffed, "The idea that it's a slippery slope and soon babies could be born to help sick parents or family friends is illogical." The reason, he argued, was that "there are only enough stem cells in cord blood to treat a certain size of child." In fact, however, the ailing Swiss child turned out to be too big, and the donor infant, having failed to provide enough cord blood, was subjected to a painful bone-marrow extraction as well.

And what if genetic tests find disease-free embryos of both sexes? Why not let parents pick the sex? "If they're already going through all the intervention," asks a New York fertility doctor, "Who are we to tell them, if the technology exists, that we can't do it?" According to the GPPC survey, most U.S. clinics that offer PGD have no policy against nonmedical sex selection, and 42 percent have done it. Dr. Jeffrey Steinberg, an American IVF entrepreneur, says his clinics have done PGD for about 2,000 couples, and "85 to 90 percent of those couples have done it simply for gender selection." Two years ago, when GPPC asked Americans whether they approved of PGD for sex selection, 40 percent said yes.

Many clinics purport to offer sex selection only to "balance" families. Originally that meant that if you had three girls and no boys, you could exclude female embryos. But "balance," like other terms, is evolving. In New York, says Steinberg, "Couples have maybe one girl and want a boy," or vice versa. A Canadian mother of two boys and a girl says she came to Steinberg because "we were desperate to have another girl and our daughter really wanted a sister. It was important for us to balance our family."

If PGD were evil, it would be easy to head off such abuses by banning it. But it's not. PGD prevents hellish diseases. In those cases, you have to say yes. And once you start saying yes, it's hard to say no. That's why they call it a slippery slope.

A version of this piece appears in the Washington Post Outlook section.

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William Saletan is Slate's national correspondent and author of Bearing Right: How Conservatives Won the Abortion War.
Photograph of an embryo on Slate's home page by Digital Vision.
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