Marrying your cousin increases birth defect risk, but only slightly

New research published today in The Lancet confirms that the two main factors associated with an increased risk of babies being born with a birth defect are being born to an older mother or to parents who are blood relations.

Consanguinity The study found that children born to parents who are first cousins are more than twice as likely to suffer from birth defects. This is comparable to the risk faced by children born to mothers over age 35, the study shows.

The UK-based Born in Bradford study is the largest of its kind, drawing on data collected from almost 14,000 babies born between 2007 and 2011 and their families.

Researchers collected lifestyle and clinical data from children born with congenital anomalies from both consanguineous (blood-related) and non-consanguineous relationships, and took into account factors such as maternal age and socioeconomic status.

The authors found that cousin marriages and maternal age were associated with a significant increase in risk of birth defects. The Bradford study was unusual in that a large number of the children were born into Pakistani families, which traditionally encourage marriage between cousins. Accounting for other factors such as maternal age and socio-economic status, the authors calculated that 31% of all anomalies in children of Pakistani origin could be attributed to consanguinity.

Other studies in the last 20 years have considered consanguinity as a cause of birth defects, but these studies weren’t able to rule out other potential risk factors, particularly the effects of deprivation.

The authors noted that while there was an increased risk, the absolute risk of birth defects remained low. They recommended: “clear and accessible information about the risks of consanguineous unions and congenital anomaly should be communicated to couples concerned and widely disseminated to local communities.

“The advice should be provided in a culturally sensitive way to promote discussion and improve awareness about the risks of congenital anomaly associated with consanguineous relationships.”

While banned in parts of Asia and many US states, marriage of first cousins is legal in New Zealand.

The Science Media Centre collected the following expert commentary. Feel free to use these quotes in your reporting. If you would like to contact an expert, please contact the SMC (04 499 5476;

Associate Professor Andrew Shelling, Faculty of Medical and Health Sciences , University of Auckland, comments:

“Cousin marriages (consanguinity) raise a number of different responses among medical researchers and specialists. This study confirms what has previously been known, that babies born to couples that are related, definitely leads to increased rates of babies born with health problems. The increased rate of problems is relatively small, and in cousins, it is not much different to those babies born to unrelated individuals.

“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 long standing 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 sensitively.

“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.”

Prof Hamish Spencer, Director of the Allan Wilson Centre for Molecular Ecology and Evolution, and Professor of Zoology at the University of Otago, has previously researched consanguineous marriage. He comments:

“The risk of birth defects in children whose parents are first cousins is often exaggerated in the public’s mind. Most such babies will be fine. All the same, there is an increased occurrence of birth defects in such babies.

“This new study, looking at babies born in Bradford, England, between 2007 and 2011, confirms that the risk is approximately twice that of the background risk (i.e., the risk to children of unrelated parents) and shows that it is not affected by poor living conditions. I would say that this increase can sound large (“twice the risk”) or small (“an increase of only about 3%”) depending on your viewpoint. One can also note that the increased risk is comparable to that to children of older mothers, something that concerns society much less.

“The study also confirms that the frequency of birth defects is higher in babies of Pakistani origin, presumably because of a tradition of marrying relatives in the Pakistani community. The authors recommend (and I strongly agree) that awareness of the risks to the children of cousin marriage needs to be increased, but in a culturally sensitive way. There are significant public-health consequences in places with higher rates of birth defects.”