Most girls who survive childhood cancer go on to become mothers, according to a major study. But boys who undergo treatment will find their chances of becoming a father significantly reduced.

The findings suggest that modern forms of chemotherapy do not rob girls of the chance of having a child, as many drugs did in the past.

Researchers found that 70 per cent treated with chemotherapy go on to become pregnant before the age of 45, only slightly fewer than the 80 per cent of healthy women who conceive by the same age.

For men, however, the research is less encouraging. Only 50 per cent of boys treated with chemotherapy in childhood go on to make a partner pregnant, compared to 80 per cent of most men.

The findings, published yesterday in the Lancet Oncology medical journal, may lead to changes in the way child cancer patients are treated.

Experts said some invasive measures taken to preserve a child’s fertility may not be required in all patients – and should instead be targeted at those most at risk.

Around 4,000 children are diagnosed with cancer every year in the UK. Medical advances mean that 80 per cent can expect to survive into adulthood.

But improved survival rates have led to a new focus on the far-reaching impacts of treatments.

Growing awareness of the long-term side effects of radiotherapy, for example, has led to its replacement with intensive chemotherapy for many childhood cancers.

The study, led by doctors at the University of Washington in Seattle, compared 11,000 childhood cancer survivors with 3,500 of their healthy siblings.

The patients had each received one of 14 common chemotherapy treatments before they were 21, between 1970 and 1999.

After eight years of follow-up, 38 per cent of survivors had either conceived or fathered a child.

The researchers calculated that if they were followed until the age of 45, 70 per cent of women would have conceived and 50 per cent of men would have made a partner pregnant.

The experts point out that the study was only conducted on child cancer patients, many of whom had not yet hit puberty.

Among adults who have already started puberty, chemotherapy may have a larger impact on their fertility.

However, the rates of pregnancy among female cancer survivors significantly dropped if they waited to start a family after the age of 30, possibly because chemotherapy accelerates the natural depletion of eggs.

Among men, experts think chemotherapy reduces sperm count. Adults about to undergo cancer treatment are often offered fertility treatment, such as freezing eggs or sperm.

For children, fertility measures are more complicated. New techniques are being developed, including freezing a part of a girls’ ovary or taking a boy’s stem cells, but doctors are far more keen to avoid damage to fertility in the first place.

Study leader Dr Eric Chow, of the Fred Hutchinson Cancer Research Centre in Seattle, said the results were encouraging but added: ‘We, as paediatric oncologists, still need to do a better job discussing fertility and fertility preservation options with patients and families before starting cancer treatment.

‘In particular, all boys diagnosed post-puberty should be encouraged to bank their sperm to maximize their reproductive options in the future.’