An unrelated couple has an approximately 3 percent to 4 percent risk of having a child with a birth defect, significant mental retardation or serious genetic disease, the researchers said. They found that close cousins face an additional risk of 1.7 percent to 2.8 percent.
That is lower than many people, including family doctors, believed to be the case, said one of the researchers, Robin Bennett.
"The commonsense point of this is that there is a definite risk, but the risk is rather small," said another one of the researchers, Dr. Arno G. Motulsky, professor of medicine and genome sciences at the University of Washington.
The analysis of six already published studies appears in the April issue of the Journal of Genetic Counseling. The six studies involved thousands of related couples.
Marriage of first cousins is illegal in 30 states and is stigmatized in many Western cultures, but that is not the case in other places, particularly the Middle East, Asia and Africa. In some countries, up to 60 percent of the population is related by blood, and cousin marriages are preferred to unions of unrelated couples.
Immigrants are continuing that tradition in the United States, so doctors need to be better informed of how to treat and advise related couples, Bennett said.
"There have surely been lots of terminations of pregnancies because of misconceptions about the actual level of risk," she said.
Researchers stressed that it is impossible to calculate the risk with precision because so many factors are involved. Risks vary among ethnic groups, and the family history and closeness of the relation may also play a role, for example.
The researchers said couples should have a thorough medical family history taken before conception to see whether they have any similar recessive genes that could show up in a child as illness.
Relatives could both be carriers of the recessive gene that causes cystic fibrosis, for example, but not have any symptoms. A child who gets the recessive gene from both parents is more likely to get the disease.
"What surprised us is that you don't really need to be doing much more than you already are," Bennett said. "Many of these tests are routine now for everyone."
The study's recommendations are the result of a two-year evaluation by genetic counselors, doctors and epidemiologists assembled by the National Society of Genetic Counselors, based in Wallingford, Pa.
Funding for the study came from the society and the Department of Health and Human Services.