Having a baby with a first cousin more than doubles the risk of congenital problems such as heart and lung defects, cleft palettes and extra fingers, according to the largest study of its kind in the UK.
The study looked at 11,300 babies born to a range of ethnic groups in Bradford, West Yorkshire, which has higher levels of blood marriage in the city’s large Pakistani community. The study is part of the wider Born in Bradford study, one of the biggest UK studies of its kind which charts a group of more than 13,000 children born in the city between 2007 and 2011.
Although the absolute risk is still considered very small, meaning that in general the majority of babies are unaffected, first cousin marriages greatly increase birth defects and the chance of a baby dying early.
The national rate for congenital defects is around 166 in every 10,000 babies, but in Bradford this nearly doubles to about 306 per 10,000 births.
Babies of Pakistani origin also account for the highest rate of infant death in England and Wales and congenital problems the most common cause in children younger than 12.
While 37% of first cousin marriages in the study were between couples of Pakistani origin, only 1% were made up of white British couples.
The researchers also found an expected increased risk of birth defects in babies born to white British women over the age of 34.
Widespread in many places across globe
First cousin marriage - known as cosanguinity - is legal in the UK and Europe, but not in some US states. It’s estimated to account for more than 20% of all marriages worldwide - more than a billion people - and both Albert Einstein and Charles Darwin were married to first cousins.
The practice is still socially embedded in many countries and groups around the world including in the Middle East and North Africa. But the higher risks of congenital problems in babies born to close relatives is well established.
Eamonn Sheridan, a clinical geneticist from Leeds University and lead author of the study, said it was the first study to definitively deal with the issue of other contributing factors such as age and obesity.
“Prior to this there was a lot of talk but little clarity. We looked at several contributory factors including ethnicity, age, health behaviours like smoking and obesity and disentangled them, which hasn’t been done before. What this does is provide very clear evidence-based research that can be used by health professionals.”
Although deprivation has been associated with an increased risk of babies born with congenital defects, and a higher number of Pakistani women are likely to earn less and live in poorer areas than their white counterparts, this doesn’t account for lower levels of genetic defects in babies born to groups in similar situations - for example in Bangladeshi women - the researchers said.
However, it was suggested in a 2011 paper that although research suggests more congenital defects from first person marriage in Pakistani communities, birth abnormalities in Pakistan were also more common because of some genetic and non-genetic causes.
Professor Alan Bittles, from Murdoch University in Perth, Australia, also said that looking at the figures alongside a baseline of disease prevalence - by comparing the total increase in congenital defects in children born to first cousin parents with those in children born to unrelated Pakistani parents - gave a less alarmist picture.
There has been much debate about first cousin marriages in the UK and cultural sensitivity around the practice. Some have called for a ban on such close relatives marrying. But others have highlighted the need for more public health campaigns, genetic testing to flag up potential issues ahead of arranged marriages and a register of people who carry genetic diseases.
The Lancet paper said higher levels of education about pregnancy halved the risk of babies born with genetic defects across all ethnic groups.
“In Bradford we’ve launched Every Baby Matters, a large-scale programme to help,” Sheridan said.
“Public health campaigns often focus on how old mum is, for example, but we clearly need another approach that includes the risks of cousin marriage. We’re engaging with the community and setting up clinics. It’s too early to know what the results are but what we do know is that people came back time and again.
"Bradford provides a model and instead of this idea that it’s an issue no-one wants to talk about, we’ve found people are interested and want to find out more. What we can’t do is legislate for how people marry.”
Neil Small, Professor of Health Research at the University of Bradford and co-lead author, said, “Clear and accessible information on these small but significant avoidable risks should be widely disseminated to local communities and be included as part of antenatal counselling and in the planning of healthcare services.”
The Born in Bradford study was set up in response to a report that investigated why Bradford had one of the highest rates of baby deaths in England and Wales.