The new report says that genetic counselors should advise cousins who want to have children together in much the same way they advise everybody else and that no extra genetic tests are required before conception.

The guidelines urge counselors to take a thorough family history and, as they do for all clients, look for any diseases that might run in the family or in the clients' ethnic groups and order tests accordingly. During pregnancy, the woman should have the standard blood tests used to screen for certain neurological problems and other disorders and an ultrasound examination.

Their children should be tested as newborns for deafness and certain rare metabolic diseases -- tests already given to all newborns in some parts of the country. These are among the conditions that may be slightly more likely to occur in children whose parents are cousins. Some of the metabolic problems are treatable, and children with hearing losses do better if they get help early in life.

Dr. Motulsky said that the panel of experts began working on the cousin question about two years ago after a survey of counselors found a lot of variability -- and misinformation -- in the advice given to people who wanted to know whether cousins could safely have children together.

The president-elect of the National Society of Genetic Counselors, Robin L. Bennett, who is a co-author of the report and a genetic counselor at the University of Washington, said: ''Just this week I saw a 23-year-old woman whose parents were cousins, and she was told to have a tubal ligation, which she did at the age 21, because of the risk to her children. And there's no risk to her children. People are getting this information from small-town doctors who may not know the risk, don't have access to this information and just assume it's a big risk.''

The young woman hopes to have the operation reversed, Ms. Bennett said.

The article in the geneticists' journal includes a personal account from a woman who said she had lived with her cousin for six years, ''and we are madly in love.'' When she became pregnant, she said, her gynecologist warned that the child would be sickly and urged her to have an abortion. A relative predicted that the baby would be retarded. She had the abortion, she said, and called it ''the worst mistake of my life.''

When she learned later that the increased risk of birth defects was actually quite small, she said, ''I cried and cried. ''