Published July 16, 2025
What are the Health Risks of Marrying Cousins?
The tabling of a private member’s bill in the UK parliament that would make cousin marriage illegal has raised the issue in public discourse.
What is cousin marriage?
Cousin marriage, or consanguinity, is a term colloquially used to describe a marriage between first cousins—the children of two siblings. In clinical genetics, however, a consanguineous marriage is one where the two individuals involved are either second cousins (the children of two first cousins) or closer.
What are the health risks?
The main health risk with cousin marriage is the increased likelihood of autosomal recessive disorders, such as β thalassemia and cystic fibrosis, which occur when a person inherits two copies of the recessive allele for the disorder. The closer the biological relationship between parents, the more likely it is that their offspring will inherit identical copies of one or more allele for a genetic disorder.
Consanguinity and congenital malformations
Between 3 and 5% of all newborns globally are born with a significant birth defect. A recent March of Dimes report highlighted that birth defect rates in most Arab countries exceed 69.9 per 1000 live births, which is higher than the rates in Europe, North America, and Australia, where it is below 52.1 per 1000 live births. However, the United Arab Emirates and Kuwait recorded lower rates, at 7.92 and 12.5 per 1000 births, respectively. In Oman, out of 21,988 births, major malformations were observed in 24.6 per 1000 births. The variation in birth defect rates across different countries and studies could be due to actual differences in populations, varying definitions and methods of identifying birth defects, or differences in the timeframes of the studies. The risk of birth defects in offspring of first-cousin marriages is estimated to be 2-2.5 times higher than in the general population, largely because of autosomal recessive disorders .
Consanguinity and postnatal mortality
In regions where consanguineous marriages are prevalent, the impact of consanguinity on mortality rates tends to be lower compared to areas with fewer consanguineous unions. This trend is not unexpected, considering the lack of comprehensive control for factors such as socioeconomic status, maternal education, birth spacing, and the quality of public health services in many studies on consanguinity. A recent large-scale study involving over 600,000 pregnancies and live births revealed that children born to first cousins face a 4.4% higher rate of pre-reproductive mortality than those from non-consanguineous relationships. In Saudi Arabia and Sudan, most research indicates that postnatal mortality rates are higher in children of consanguineous parents compared to those of unrelated parents. The elevated postnatal mortality in offspring of consanguineous couples may be attributed to harmful recessive genes and complex genetic factors inherited from common ancestors. Interestingly, the higher number of children typically born to consanguineous couples tends to offset the increased infant mortality rate, often leading to a similar number of surviving children between consanguineous and non-consanguineous families.
Overall, the findings from various studies provide critical insights into the genetic consequences of consanguineous marriages, offering valuable guidance for healthcare policy and practice in regions where such marriages are prevalent.