Fertility researchers said Wednesday that they have come up with a way to sort sperm that may allow couples to have babies of the sex they choose.

Doctors at the Genetics & IVF Institute said sperm cells to be used in artificial insemination can be sorted by the amount of DNA they contain.The only difference between male-producing Y chromosome sperm and female-producing X chromosome sperm is that sperm carrying a Y chromosome have about 2.8 percent less genetic material.

Using a DNA detector, researchers were able to sift sperm to produce samples in which 85 percent of the cells had an X chromosome, the institute's doctors said in a statement. If they targeted Y-bearing sperm, the result was a sperm sample in which 65 percent of the cells contained a Y chromosome.

The research is published in the journal Human Reproduction.

In data reported from the institute's ongoing trials, 29 pregnancies have been initiated after separation for the X-bearing sperm. So far, nine patients have given birth to 11 healthy baby girls, the institute said.

A study of couples wanting boys produced results consistent with what the sperm sorting would predict, lead researcher Dr. Edward Fugger said. Exact results were not released.

Sex-selection techniques have been marketed and promoted before. But Dr. Alan DeCherney of the University of California at Los Angeles told The New York Times: "Nothing worked until now."

The researchers refined techniques developed for sorting the sperm of farm animals. Fugger said of the more than 400 animals born through sperm sifting, none were born with defects.

Dr. Barry Zirkin of Johns Hopkins University said he still was not convinced about the safety of the method, which involves staining sperm cells' DNA with a fluorescent dye and shining laser light on the sperm. DeCherney also cautioned about the ethical implications of tinkering with sperm.

When it comes to sex selection, he said, "most people feel this is tampering with nature."