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LONDON (Reuters) - Children whose parents are cousins run more than double the risk of being born with a congenital abnormality, although the overall rate of such birth defects remains low, according to new research findings.
A large study in a British city with a large Pakistani community, where marriage between blood relatives is fairly common, found that so-called consanguineous parents accounted for more than 30 percent of birth defects in babies of Pakistani origin.
Researchers said the findings were important evidence for use in educating populations that accept and sanction cousin marriages - including Amish, Kurdish, Romany and other relatively closed communities - about the potential risks for children's health.
Birth defects, also known as congenital abnormalities, can range from relatively minor problems such as extra fingers or toes through to more life-threatening problems such as holes in the heart or brain development disorders.
Experts estimate more than a billion people worldwide live in communities where blood-relative marriage is a cultural norm.
"Whilst consanguineous marriage increases the risk of birth defect from 3 percent to 6 percent, the absolute risk is still small," said Eamonn Sheridan, a senior lecturer in clinical genetics at the University of Leeds who co-led the study and presented its results at a briefing in London.
He added that this still means 96 percent of blood-relative couples are likely to have babies with no birth defects: "It's important to note that the vast majority of babies born to couples who are blood relatives are absolutely fine."
Called "Born in Bradford" or BiB, the study was the largest of its type ever conducted and looked at more than 11,300 babies in the northern English city of Bradford between 2007 and 2011.
The researchers found the overall rate of birth defects in the BiB babies - which included largely white British and Pakistani mothers but also other ethnic groups - was approximately 3.0 percent, nearly double the national rate of around 1.7 percent.
Among the Pakistani subgroup, they found 77 percent of babies born with birth defects were born to parents who were in blood-relative marriages.
Neil Small, a professor of Health Research at the University of Bradford who worked with Sheridan on the study, said he hoped the robust evidence provided by the study would prove useful in raising awareness among communities across the world.
"At the heart of all this are children who are born with often very distressing illnesses that can create both misery in themselves and anguish in the families," he said.
"Many of these things are preventable and we hope that what our study does is contribute to a debate that means in the future, some of them will be."
Responding to the study's findings, Hamish Spencer, a professor of zoology at New Zealand's University of Otago who has previously researched consanguineous marriage, said they were important because there are significant public health consequences in places with higher rates of birth defects.
"Awareness of the risks to the children of cousin marriage needs to be increased but in a culturally sensitive way," he said in an emailed comment.
Reporting by Kate Kelland; editing by Patrick Graham