PHILADELPHIA -- The risk of birth defects in children born to couples who are first or second cousins isn't as high as many experts had believed, according to a study that sheds new light on a practice that is stigmatized in many Western cultures.
The study released Wednesday says married cousins are still more likely than unrelated couples to have children with a birth defect, significant mental retardation or serious genetic disease.
An unrelated couple has about a 3 percent to 4 percent risk of having a child with such problems. But for close cousins who are married, that risk jumps only 1.7 percent to 2.8 percent, the study said.
Researcher Robin Bennett said that is lower than many people, including family doctors, believed to be the case.
"The commonsense point of this is that there is a definite risk, but the risk is rather small," said one of the researchers, Dr. Arno G. Motulsky, professor of medicine and genome sciences at the University of Washington.
Marriage of first cousins is illegal in 30 states and is taboo 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.
Bennett said doctors are confronted with new challenges about advising and treating related couples as more immigrants carry on such traditions in the United States.
"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 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.
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.
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