A method for earlier genetic testing.

After plucking a single cell from an eight-celled embryo conceived through in vitro fertilization (IVF), geneticists have successfully determined whether the embryo carries the genetic mutation that causes cystic fibrosis.

The advance offers a means of detecting some genetic diseases much earlier than is possible using amniocentesis or chorionic villus sampling, two widely used techniques for diagnosing common genetic abnormalities. Unlike these tests, the new technique -- called preimplantation diagnosis -- is performed before a pregnancy begins.

A team led by Alan H. Handyside of Hammersmith Hospital in London, England, removed a cell from each of five embryos that had been conceived in a laboratory dish three days earlier. Both parents were carriers of the mutation responsible for cystic fibrosis and therefore had a one-in-four chance of having a child affected by the disorder, which clogs vital organs such as the lungs with infection-prone mucus.

Handyside collaborated with a group led by Mark R. Hughes of the Baylor College of Medicine in Houston to examine the five isolated cells for mutations. They found that two contained two copies of the mutant gene responsible for cystic fibrosis, one was a carrier of the disorder, and two others were unaffected.

Handyside's group implanted a carrier and an unaffected embryo into the womb of the mother. One developed, and the woman gave birth to a healthy baby, both teams of researchers report in the Sept. 24 NEW ENGLAND JOURNAL OF MEDICINE.

Preimplantation diagnosis "represents substantive progress" in genetic testing, Joe Leigh Simpson and Sandra Ann Carson of the University of Tennessee in Memphis comment in an editorial accompanying the report. Because the technique involves embryos that have not yet been implanted in the mother's womb, they write, couples may avoid the agonizing decision of whether to abort an affected fetus.

Simpson and Carson caution, however, that preimplantation diagnosis is technically difficult. They also question whether the technique's $2,000 price tag makes it a practical way to conceive a baby -- especially since it can only be administered to embryos conceived through the already costly IVF process.