File photo: A scientist works during an IVF process. Picture: AP

London - British fertility clinics are accused of a cover-up over the number of women developing a painful and potentially fatal side effect of IVF.

Doctors are legally obliged to report "severe" cases of ovarian hyperstimulation syndrome, when IVF drugs cause women’s ovaries to expand dangerously.

For at least six years, they have told watchdogs that between 16 and 60 women have been affected annually.

But figures uncovered by the Mail show about 800 women every year have to be taken to hospital after being injected with IVF drugs and developing the condition.

In one year, clinics reported just 16 cases of severe OHSS, which causes extreme back pain, stomach swelling, nausea, breathlessness and, in extreme cases, death. However, there had been almost 700 emergency hospital admissions for the condition – more than 43 times the number declared by IVF doctors. There has also been a huge increase in the number of women suffering with the OHSS, the Mail has found. At least four women in the UK have died as a result of it.

The Human Fertilisation and Embryology Authority said it was asking urgent questions of IVF clinics. Chairman Sally Cheshire told the Mail that it was "very concerned" fertility doctors might be under-reporting cases of OHSS.

MPs called for an overhaul of legislation to protect women. The revelation comes on the third day of a major investigation by the Daily Mail into the fertility industry.

We have revealed how IVF clinics convince women on low incomes to donate eggs in return for cash or free treatment.

Clinics also give women false hope of delaying motherhood by exaggerating their success rates with frozen eggs. The HFEA has launched a full inquiry into the clinics involved.

Women who have IVF are at risk of developing OHSS because of the hormone injections they have to take to stimulate egg production can make their ovaries go into overdrive. It is only possible to develop the condition as a result of fertility treatment.

In severe cases, OHSS will cause the ovaries to expand to more than 12cm – about five times their usual size. Excess fluid can accumulate in the abdomen, or around the lungs and heart, causing severe pain.

This can lead to circulation and kidney problems, with some women having to be treated in intensive care with intravenous infusions.

Clinics are obliged to report cases of patients having to be hospitalised because of severe or critical OHSS. But experts say the term ‘severe’ is vague.

Last year, there were 60 cases of severe or critical OHSS reported by UK clinics. But the NHS recorded 865 cases of women with OHSS going to hospital – and 836 were so serious they were emergency admissions. In 2010, clinics reported 16 cases of OHSS to the HFEA. But that year there were 691 emergency admissions. In total, IVF clinics have reported 256 cases between 2010 and 2015. But over this time, there have been 4 792 admissions, with 4 587 classed as emergencies.

As IVF has become increasingly popular, cases have risen. In 1995/96, there were a relatively low 404 emergency OHSS admissions.

IVF expert Dr John Parsons, a retired consultant at King’s College Hospital, said: "The definition of 'severe' is so vague. If the HFEA wants to get a true picture of OHSS it should insist that clinics report all cases when the patient has to be hospitalised." 

Labour MP Siobhain McDonagh, who sat on the Women and Equalities Select Committee and has raised the issue of OHSS in parliament, said: "The Mail has done a huge service by uncovering these figures. This is ground-breaking.

"The difference between the NHS statistics and the figures reported are incredible. The law needs to specifically mention the need for doctors to protect the welfare of women."

Tory MP Dr Sarah Wollaston, who was chairperson of the health select committee, said: "We need to look carefully at the reasons for the discrepancies."

The HFEA said that in some cases, women could have been admitted with mild or moderate OHSS, which might explain why the NHS data is higher.