More American doctors are using frozen donor sperm as a means of impregnating women who want children but can't have them because their husbands are infertile.

"If you can bake a cake, you can freeze donor sperm," Dr. Armand M. Karow Jr. of the Medical College of Georgia at Augusta told the 145th national meeting of the American Association for the Advancement of Science. "It's exceptionally simple if you just follow the recipe."

Last year in the United States, possibly 20 percent of the 10,000 children born using the artificial insemination method were conceived from donor sperm that had been frozen, Dr. Karow said. Ten years ago, there were no artifical inseminations from frozen sperm.

When sperm is frozen, tests can be run to make sure it doesn't contain defective genes and won't transmit contagious diseases that could harm or kill both the mother and child.

"Artificial insemination with thawed semen results in far fewer miscarriages and in fewer birth defects than one finds in the general population," said Dr. Karow.

Another reason for freezing sperm is to store it for up to 10 years without altering of genes. This permits women who want to receive donor sperm to shop for the best possible match. Ethnic origin, height, weight, build, skin, eye and hair color can be matched ahead of time.

The process of freezing sperm involves treating it with glycerol to protect it, placing it in plastic straws, and freezing it with liquid nitrogen at almost 400 degrees below zero.

There is, however, at least one drawback to frozen sperm. It does not produce pregnancies as rapidly or as often as fresh sperm does, in part because thawed sperm does not move as quickly to fertilize the eggs of a woman receiving the sperm.

Dr. Karow said freezing sperm could enhance the development of techniques to allow parents to select the sex and mental makeup of their children.

"We can even imagine a normal couple selecting semen from a panel of famous people and doing home insemination." Dr. Karow said, raising the possibility that some people might seek to have children with preselected traits unlike their own.