Where Do Babies Come From?

By William Saletan
Sunday, September 17, 2006

"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. In thousands of cases each year, the story now includes pre-implantation genetic diagnosis (PGD), 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 (GPPC), suggests that 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 thirties but usually arrives much later. According to the forthcoming issue of the Journal of Clinical Oncology, PGD has also been used to wipe out colon cancers -- which don't develop until age 45 to 55 and are treatable if detected early.

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

Probability and life expectancy 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 all male embryos, which are more likely than females to develop the disorder. 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, which is highly manageable, is 20 percent .

Once you screen for one gene, it's tempting to screen for others. The woman who's targeting arthritis added that gene to a planned test. Another patient, described in the same article, set out to scan his embryos for colon cancer and ended up chucking two more because of 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 introduced 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 for the benefit of future kids, why not do it for kids already here? Some embryos are particularly suited to donating cord blood to a sick sibling; nearly one in four American PGD clinics has sorted embryos for this purpose. Originally, this was done for diseases such as Fanconi anemia, in which the embryo could be checked for its own sake 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 say they've used it to identify which embryos could be useful donors, even when the test offers the embryos no benefit.

Once you start treating a child like a tissue bank, it may be hard to stop. Last month, commenting on a Swiss couple's use of 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, is that "there are only enough stem cells in cord blood to treat a certain size of child." In fact, the ailing Swiss child turned out to be too big, and the donor infant, having not provided enough cord blood, was subjected to a painful bone-marrow extraction.

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. Jeffrey Steinberg, an American IVF entrepreneur, says his clinics have performed 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, Steinberg says, "couples have maybe one girl and want a boy," or vice versa. A Canadian mother of two boys and a girl says she went 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 abuse 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.

William Saletan covers science and technology for Slate, the online magazine at

© 2006 The Washington Post Company