Over the last five years, a single NHS trust has spent $3.6 million treating the genetic defects of children born from related parents. Responding to FOI requests from journalist Lewis Brackpool, Birmingham Women’s and Children’s NHS Foundation Trust said it treated 1,559 patients between January 1, 2020 and June 1, 2025 who were diagnosed with a “family history of consanguinity”.

Bradford Teaching Hospitals NHS Foundation Trust said it was unable to provide figures, claiming its data could not distinguish between genetic conditions caused by consanguinity and those that were not. In contrast to Birmingham, which relied on a single diagnostic code, Bradford said its coding system lacked the specificity to attribute cases to parental relatedness.

It went on: “Identifying relevant cases would require a manual review of individual patient records, which would exceed the appropriate cost and time limits under the Freedom of Information Act.”

Data from Born in Bradford (BiB) from 2023 showed that in three inner-city wards, 46% of mothers from the Pakistani community were married to a first or second cousin. Cousin marriage has been cited as the cause of 53% of all South Asian infant deaths from genetic disorders in the city. There is not yet data for how many cousin marriages there are in Birmingham but studies suggest that 20% of infant deaths in the city are the result of genetic complications from birth, the risk of which is doubled by having parents who are related.

These new figures come as the Marriage (Prohibited Degrees of Relationship) Bill works its way through Parliament, with its second reading due this week. Conservative MP Richard Holden introduced the bill in December, arguing that cousin marriage poses “risks to health, freedom [and] the cohesion of our society”. In February this year, the BBC reported on new data from Born in Bradford showing that children of first cousins were more likely to have speech and language difficulties, more likely to need more hospital appointments, and less likely to “reach a good stage of development”. A 2011 study showed that the likelihood of infant mortality increases substantially if the parents are first or second cousins.

In response to the bill, Independent MP Iqbal Mohamed defended the practice in the House of Commons. He argued that the Government should not outlaw cousin marriage and said that rather than “stigmatising” it, related couples who wish to have children should be given advanced genetic screenings. Mohamed also said that the practice can “help build family bonds”.

YouGov polling from May showed that 77% of Britons want first-cousin marriage outlawed. While only 9% of Britons think the practice should remain legal, 39% of the British Pakistani and British Bangladeshi communities believe the same.