First cousins who marry increase the risk of giving birth to a baby with birth defects including defects affecting the heart or lungs, a British study suggests.

Researchers looked at birth anomalies in Bradford in the north of England where there is a large Pakistani community. In that community there's a high level of consanguineous marriage — matrimony between blood relatives. 

In Wednesday's online issue of the Lancet, they concluded that consanguinity accounted for 31 per cent of all anomalies in the Pakistani community's children. The risk of birth defects was double that in those of white British origin.

si-wedding-rings-220-cp-is

There are also cultural, religious, social and economic issues that underpin marriage within families. (iStock)

Of the more than a total of 11,000 babies in the study, 386 or 3 per cent had a congenital anomaly, such as heart and lung defects, cleft palates and Down syndrome. The children were born between 2007 and 2011.

Rates of congenital anomaly were 305.74 per 10,000 live births in the Pakistani community compared with a national rate of 165.90 per 10,000.

Socially sensitive study

Overall, 18 per cent of babies in the study were offspring of first cousins. Less than one per cent of babies of white British origin were born to first cousins compared with 38 per cent in the Pakistani group.

"Our findings confirm that the offspring of consanguineous unions have an increased risk of congenital anomalies, which is independent of deprivation," Eamon Sheridan of St. James's University Hospital in Leeds, UK, and co-authors concluded.

"Couples contemplating such unions should be advised of these risks; however, advice should be given with sensitivity and cultural awareness."

Providers of pediatrics, obstetric and genetic services should consider the increased needs, the study's authors suggested.

Unlike in previous research, maternal smoking, alcohol consumption, and obesity were not identified as risk factors for birth defects in this study. The risk of birth anomalies did increase for all  mothers over age 34. The researchers acknowledged that self-reporting of lifestyle factors might not be reliable.

A high level of maternal education was protective regardless of ethnic origin, roughly halving the risk of having a baby with a congenital anomaly.

A journal commentary accompanying the study noted that the 3.6 per cent absolute increase in congenital anomalies in children of Pakistani first cousins in Bradford compared with those of unrelated Pakistani parentage and reduces "the emotional loading for patients and the general public alike."

Alan Bittles of the Centre for Comparative Genomics at Murdoch University in Perth, Australia, said most health investigations of consanguinity don't acknowledge the cultural, religious, social and economic issues that underpin marriage within families.

The study lacked information on biraderi or brotherhood membership — traditional male lineages for kinship, occupational and social networks, within the UK Pakistani community, Bittles said.

"Sheridan and colleagues deserve major credit for their complex, time-consuming, and socially sensitive study," Bittles said.

Under Canada's Marriage Act, marriage is not allowed between those who are "related lineally, or as brother or sister or half-brother or half-sister, including by adoption," according to the Justice Department's website.

It's estimated more than one billion people worldwide currently live in communities where consanguineous marriages are common, the journal said.