The number of girls at risk of FGM is hard to estimate

In today's front page article on the campaign to end female genital mutilation (FGM), the Guardian said that there are an estimated 24,000 girls in England and Wales who are at risk.

So where does the number come from, and what is meant by 'at risk'?

We don't have all the data we need to calculate the risk

The number comes from a report published in November last year by a coalition of groups including the Royal College of Obstetricians and Gynaecologists (RCOG), trade unions and others. This in turn referenced a 2007 report by the Foundation for Women's Health, Research and Development (FORWARD).

In order to estimate the number of girls at risk, the report's authors collected information on the proportion of girls undergoing FGM in 29 countries and used this to categorise the countries according to the level of risk to girls. To estimate the risk to girls living in the UK, it assumed that the risk of FGM is the same for children in the UK as it is in the country they or their mother migrated from.

And that is a big assumption. We know very little about the relationship between FGM practices abroad and here. As the report itself says:

"...there are very few data on the effect of migration on the practice. One study suggested a lower prevalence of FGM among young Somalis in London than the population average in Somalia."

For this reason the report recommends that further work be done to produce better estimates of risk in the affected populations.

The absence of this information meant that its estimate of the number of girls at risk relied on that assumption.

There are different types of FGM

The report looks at three categories of genital mutilation. These are based on the World Health Organisation's classification system. Under this system the amount of tissue that is cut generally rises from Type I to Type III, with some exceptions

It finds that 24,000 girls under 15 years of age are at high risk of FGM Type III, which is particularly likely to lead to serious and long-term health complications. It also finds that about 9,000 girls are at high risk of FGM Type II or I, and about 15,000 are at a medium risk of this.

We don't know what the situation in 2014 is

The estimate is partly based on information from the 2001 census, which gives details of the number of girls living in the UK that year who were born, or whose mothers were born, in countries with high rates of FGM.  But the proportion of UK residents who come from the countries in question, or the birth rate for women belonging to this group, is likely to have changed since then. This will have an impact on the number of girls affected.

We don't know how many girls in the UK are at risk

According to the RCOG report, information on the numbers of girls at risk of FGM is not systematically collected by health or social service providers, and nor are the numbers of girls who have been treated following mutilation. So official sources cannot tell us whether the number at risk is higher or lower than 24,000; we just don't know.