PGD uses the same techniques as prenatal diagnostic testing, but is done at the moment of fertilization, before doctors have implanted the embryo in a woman's womb. That means fertilized embryos testing positive for unwanted genetic traits can easily be discarded in the search for a child with specific genetic code.

A U.S. couple gave birth to a PGD-screened child in October. Adam Nash was tested as an embryo to see if his DNA matched that of his older sister, who needed the blood from his umbilical cord to help fight her rare case of Fanconi anemia. The Nashes' decision is the first known case in which parents used genetic testing to select a baby on the basis of his ability to save a sibling's life, setting what many bioethicists are calling a worrisome precedent for other desperate parents of ailing children.

Overall, PGD has been performed in over 40 genetics centers around the world. Nearly 400 children have been born from approximately 2,000 cycles of testing performed for genetic or chromosomal disorders. That means roughly eight of every ten embryos were not implanted and carried to term.

Proponents of the new technique say PGD is simply a method of helping couples in a painful predicament who desire to have children of their own.

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"There is very little we can actually do once an embryo or fetus has been diagnosed with a genetic condition, ...