Cousins’ Offspring Risk Called Overblown

Children born to couples who are first or second cousins have a lower risk of birth defects than is commonly believed, but it still can be nearly twice as high as usual, according to a study released Wednesday.

An unrelated couple has an approximately 3% to 4% 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% to 2.8%.

That is lower than many people, including family doctors, believed to be the case, said Robin Bennett, one of the researchers.

“The common-sense point of this is that there is a definite risk, but the risk is rather small,” said Dr. Arno G. Motulsky, another of the researchers and a professor of medicine and genome sciences at the University of Washington.

The analysis of six already-published studies appears in this month’s issue of the Journal of Genetic Counseling. The six studies involved thousands of related couples.

In some countries, up to 60% of the population is related by blood, and marriages between cousins are preferred to unions of unrelated couples. Immigrants are continuing that tradition in the United States, so doctors need to be better informed about 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.

Funding for the study came from the National Society of Genetic Counselors and the Department of Health and Human Services.