A new report suggests that no local authority in England and Wales is likely to be free from the curse of female genital mutilation.

Previous estimates have suggested that hundreds of thousands of women – mainly migrants from Africa – were likely to have been cut before moving to the UK, and tens of thousands of young girls were “at risk” of having the procedure performed on them.

But FactCheck has found before that there is a huge amount we don’t know about the likely prevalence of FGM in this country.

What’s new today?

The latest study was done by academics from City University and was partly funded by the Home Office.

It estimates that there are 137,000 women and girls living in England and Wales who “who have undergone or are likely to undergo FGM”.

Some of these women are likely to be living in every region and local authority in England and Wales, with implications for local health services.

Where do the numbers come from?

The methodology is similar to other studies that have tried to estimate the scale of the problem on these shores.

First you estimate the prevalence of FGM in various countries, using data collected by Unicef.

The numbers – based on local people answering questions in health surveys – show that mutilation is a mainly African problem, although there are countries in Asia where it takes place too (graphic – Unicef):

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Contrary to what some people believe, mutilation is not just a Muslim issue: it happens in Christian and animist areas too.

But the researchers judge that other religious groups like Sikhs, Hindus and Jews do not cut girls, and they tried to filter these groups living in high-risk countries out of their estimates.

After assembling prevalence estimates for various countries, you then figure out how many women from those countries have come to live in the UK, using data from the 2011 census.

The new study takes a more sophisticated approach than previous efforts, factoring in different rates for different age groups, and removing groups likely to have skewed earlier estimates – like white and Asian women born in FGM-practising countries.

Nevertheless, it’s a top-down, statistics-based approach, and not based on hard evidence of cases where women actually report having been mutilated.

The new headline figure of 137,000 women likely to be affected by FGM is more than double the same authors’ previous estimate of 66,000, largely because the number of migrants from the danger countries has gone up considerably between the 2001 and 2011 census.

There are some weaknesses in all this data. Some of the Unicef prevalence estimates are old, dating from the late 1990s in some cases.

The immigrant population estimates come from 2011, so may underestimate the size of various groups.

It’s also difficult to know whether woman who migrate to England and Wales are representative of their communities. If they are more highly educated than average, they may be less likely to have been mutilated.

Arguably, the statistics relating to different local authorities don’t tell us much other than that there are migrants from countries where FGM is practised living across in England and Wales. As you might imagine, the prevalence was estimated to be highest in London and other cities.

What about girls born in Britain?

A huge question that this study does not claim to be able to answer. It seems fair to assume that second-generation migrant children are at higher risk if they are born into a culture where FGM is practised, but some research suggests that families are more likely to abandon it after migrating.

The authors say: “Our earlier report attempted to assess the numbers of girls born in the UK who could be described as being ‘at risk’. This is no longer appropriate.

“On the one hand, qualitative research has shown that attitudes to FGM have changed on migration and in response to community-based programmes and many families have abandoned it while on the other, there are still reports of girls living in England and Wales being subjected to FGM or threatened with it.

“In neither case, has the extent been quantified in a way which can be used in numerical estimates at a population level. Risks to girls have to be assessed through contacts with individual mothers and families.”

Are other figures available?

Last week Bedfordshire Police became the first force to use new powers to prevent British girls being taken abroad for the purposes of genital mutilation.

Police used control orders to prevent two girls travelling to Somalia for the operation.

National figures on arrests, prosecutions and convictions haven’t been made available to FactCheck at time of writing, but we understand that no one has yet been convicted of performing FGM in Britain, despite the fact that it has been illegal since 1985.

A new act of parliament raised the maximum sentence in 2003, but it was not until 2010 police referred the first case to the Crown Prosection Service.

There were 20 more between 2010 and 2013, according to the home affairs committee, who noted that France, by contrast, had achieved more than 40 prosecutions since 1979 and punished more than 100 people.

Earlier this year the first UK prosecution failed when NHS surgeon Dr Dhanuson Dharmasena was cleared by a jury of carrying out FGM after he stitched up a woman who had just given birth.

Dhanuson Dharmasena arrives at Southwark Crown Court in London

After the trial Commander Mak Chishty, the national police lead on FGM, said the lack of successful prosections “is not through a lack of vigor or effort, it’s just that the evidence hasn’t been there to support the charge”.

He added: “If you haven’t got a compliant victim, or when and where the offence occurred, or any of the complexities we need to prove beyond reasonable doubt, there can be difficulties.

“There are a lot of cultural barriers in the way of people coming forward, which we’re trying to overcome through our engagement and raising awareness.”

The Health and Social Care Information Centre also publishes experimental statistics on FGM. The latest figures show there were about 550 new cases of various kinds of mutilation recorded each month by hospital trusts across England:

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