The number of babies born with birth defects in Bradford is nearly double the national average, research conducted in the city has shown.
The study found this was largely because of marriages between first cousins in the Pakistani community.
Consanguineous marriage accounted for nearly a third of abnormalities in a study of more than 11,300 babies.
Researchers said the risk of birth defects among children of blood relatives remained small, at 6%.
The study looked at babies involved in the Born in Bradford (BiB) project, a long-term health study of thousands of people born in the city, between 2007 and 2011.
About 1.7% of babies in England and Wales are born with a birth defect such as heart problems or Down's Syndrome, but among BiB babies the rate was nearly double that, at 3%.
'Make informed choices'
The study's authors said it was the largest of its kind ever conducted and the first to include significant numbers of people in both consanguineous and non-consanguineous groups.
The study included 43 different ethnicities but the largest ethnic groups were Pakistani (45%) and white British (just under 40%).
Of 5,127 babies of Pakistani origin, 37% had married parents who were first cousins, compared to less than 1% of married couples nationally.
It is estimated that, worldwide, more than a billion people live in communities where marriage between blood relatives is commonplace.
Last year, the BiB project found cousins getting married was becoming more common among British-born Pakistanis in Bradford than a generation ago.
Aisha Ali Khan, from the city, is healthy but four of her siblings were born disabled as the result of a particular gene carried by both her parents, who are cousins.
"It works for so many people and they have fantastically successful marriages, however I would always say at least get some kind of genetic testing done," Ms Khan said.
"It's very common in Pakistan now. My cousins are really aware of all this - when they talk about potential suitors, they talk about genetic testing."
The study's lead author Dr Eamonn Sheridan, from the University of Leeds, said: "It is important to note that the vast majority of babies born to couples who are blood relatives are absolutely fine, and whilst consanguineous marriage increases the risk of birth defect from 3% to 6%, the absolute risk is still small."
Co-author Prof Neil Small, from the University of Bradford, said: "It is not our intention to counsel couples about who they choose to marry, but we do want to ensure that couples are aware of any risks so that they can make informed choices when planning their families."
There was also an increased risk for babies born to older women, which the researchers said was already a well-established link. Among the white British group, 19% of the babies with birth defects were born to women over the age of 34.
But levels of deprivation had no effect on the risk of birth defects and "higher levels of maternal education halved the risk".
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