First cousins face lower risk of having children with genetic conditions than is widely perceived
Cousins contemplating marriage or concerned about a pregnancy arising from their union have often found it difficult to get accurate information about risks to their offspring.
In a paper published in the April issue of the Journal of Genetic Counseling, a task force made up of genetic counselors, physicians and epidemiologists, among others, has evaluated the evidence about risks for offspring for first cousins and provides guidelines for counseling and advising such couples.
The task force was brought together by the National Society of Genetic Counselors. It considered recommendations for various unions of consanguineous (literally, blood-sharing) couples related as second cousins or more closely.
The consensus of the task force and those who reviewed the recommendations "is that beyond a thorough medical family history with follow-up of significant findings, no additional preconception screening is recommended for consanguineous couples." They should, of course, be offered genetic screening tests that would routinely be offered to other couples of their ethnic group.
In part because of social stigma and because marriage between first cousins is prohibited in 30 states and laws on other consanguineous relationships vary, the authors note that many such unions are kept secret.
"Because of widespread misconceptions about the actual level of risk to offspring, some of these pregnancies are terminated and other couples suffer a lot of needless anxiety," said Robin Bennett, lead author of the paper and president-elect of the National Society of Genetic Counselors. Bennett is a certified genetic counselor at University of Washington Medical Center and manages the Genetic Medicine Clinic there.
The paper's senior author is Dr. Arno Motulsky, professor emeritus of medicine and genome sciences at the UW and a pioneer in medical genetics studies.
Relatively few studies have documented actual risks to the offspring of consanguineous unions, the authors note, and many of the studies that have been done are flawed in terms of their relevance for the general population. The task force reviewed all studies published in English in the medical literature, and some additional materials.
What the authors were looking for is the additional risk of significant birth defects (mental retardation or genetic disorders) -- or risk that is more than that faced by the general population of couples. For example, for couples, if the base (general population) risk of genetic conditions is 5 percent, it's the additional risk that is important for consanguineous couples to know.
Although they emphasize that it's not possible to come up with one number for all populations of consanguineous couples, the authors estimate the additional risk to range from 1.7 to 2.8 percent for first cousin unions. From her experience in counseling, Bennett believes these numbers are far lower than most people's perception of the risk.
One reason these issues and questions have come to the fore now is that health care practitioners are seeing more cousin unions in the immigrant population coming to North America from Africa and the Middle East. In some of these societies, the authors note, cousin marriages are actually traditionally preferred and quite common. Better information and appropriate guidelines are especially needed by physicians and genetic counselors who work with these groups so that more objective and culturally respectful services can be provided.
The paper also includes guidelines for screening for the recessive genes that can produce offspring with disorders of metabolism or hearing disorders, among others. In many cases, these disorders can be treated if found early in life. In the same vein, the importance of routine, regular early childhood pediatric care, as set out in American Academy of Pediatrics guidelines, is emphasized for children of cousin unions.
And what about the laws preventing cousins from marrying? The authors note such laws may eventually change as a result of evidence about actual risks.