First cousin marriage can double risk of birth defects, finds study

BMJ 2013; 347 doi: (Published 05 July 2013) Cite this as: BMJ 2013;347:f4374
  1. Krishna Chinthapalli
  1. 1BMJ

Children born in marriages between first cousins have double the risk of congenital anomalies, a new UK study has found.1

Researchers followed 13 776 pregnancies in Bradford and found that 6.1% of children born to first cousins had congenital anomalies and that 98% of these children were born to people of Pakistani origin. This compared with a 2.4% risk of congenital anomalies in non-consanguineous marriages in the study (multivariate relative risk 2.2 (95% confidence interval 1.7 to 2.9) and a background risk of 1.7% in the UK population. The researchers found that the risk was unchanged when they controlled for socioeconomic status.

The findings are in keeping with previous research and concerns among paediatricians in Bradford over the number of autosomal recessive disorders in children there.2

At a press briefing on 3 July, Neil Small, professor of health research at the University of Bradford and a coauthor, said, “The single biggest cause of infant mortality in Pakistani babies is congenital anomalies . . . This is the first study that has been able to explore all causes of congenital anomaly in a population where there are sufficient numbers in both consanguineous and non-consanguineous groups to come to reliable conclusions.”

The Born in Bradford study, published in the Lancet, also looked at other potential contributing factors. It found that mothers of white British origin older than 34 years had nearly twice the risk as white British women 34 or younger of having children with congenital anomalies (absolute risk 3.7% versus 2.1%; relative risk 1.8 (95% confidence interval 1.1 to 3.3) but that age was not significant in other ethnic groups. Mother’s education to degree level halved the risk of congenital anomalies, and there was also a decrease in risk with maternal smoking and alcohol intake, for which the authors suggest ethnicity as a confounding factor.

Eamonn Sheridan, senior lecturer in clinical genetics at the University of Leeds and the study’s lead author, said that the two most common types of congenital anomaly accounting for the raised risk in the study were neurological abnormalities, such as microcephaly, and congenital heart disease, especially atrial or ventricular septal defects. He added, “It is important to note that the absolute increase in risk is small, meaning that only a small minority of babies born to couples who are blood relatives or older mothers will develop a congenital anomaly.”

In a linked editorial, Alan Bittles, professor of community genetics in Perth, Australia, said, “Substantial additional information about the health outcomes of intrafamilial marriage can be anticipated, especially if accompanied by exome sequencing, . . . the establishment of comprehensive disease registries and targeted community-based carrier screening programmes.”3

Elsewhere Bittles has suggested that over one billion people are married to a close relative or are children from such marriages and that health risks from consanguineous marriages have previously been exaggerated.4

Andrew Shelling, associate professor of obstetrics and gynaecology at the University of Auckland, agreed. “For some ethnicities, cousin marriage is relatively common, and we have seen increasing numbers of these communities living in New Zealand who will have a longstanding tradition of consanguinity. While these increased risks should be clearly communicated to all couples, just as we discuss other potential medical issues for parents, this will need to be done carefully and with cultural sensitivity,” he said in a statement on the new findings.

He added, “In an interesting twist, and possibly a social comment, this study shows that the risk is about the same as older women (defined as having babies over 34) having babies.”

The researchers now plan to follow up the children in the study to assess their long term health.


Cite this as: BMJ 2013;347:f4374