The new technique would be performed on a two-day-old embryo, after the fertilized egg has divided into eight cells, known as blastomeres. In fertility clinics, where the embryo is available outside the woman in the normal course of in vitro fertilization, one of these blastomeres can be removed for diagnostic tests, like for Down syndrome.
The embryo, now with seven cells, can be implanted in the woman if no defect is found. Many such embryos have grown into apparently healthy babies over the 10 years or so the diagnostic tests have been used.
Up to now, human embryonic stem cells have been derived at a later stage of development, when the embryo consists of about 150 cells. Both this stage, called the blastocyst, and the earlier eight-cell stage, occur before the embryo implants in the wall of the womb. Harvesting the blastocyst-stage cells kills the embryo, a principal objection of those who oppose the research.