The growing practice of embryo eugenics.

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.