Fears that IVF might raise women’s risk of developing breast cancer have been dispelled by a nationwide study in the Netherlands, which has found that the fertility treatment has no effect on the disease.

While no link between IVF and breast cancer has been firmly established, some scientists have worried about the potential effects of fertility drugs used to stimulate the ovaries so that eggs can be collected and fertilised. These expose the body to high levels of oestrogen, a female hormone to which some breast tumours are sensitive.

Some women who have had IVF and then gone on to develop breast tumours have blamed their condition on it. Sarah Parkinson, the late wife of the comedian Paul Merton, wrote before her death in 2003 of her belief that IVF had caused her cancer.

The new research, led by Alexandra van den Belt-Dusebout, of the Netherlands Cancer Institute, should reassure women considering fertility treatment that it does not pose a breast cancer risk.

In the study, which was presented at the American Society for Reproductive Medicine conference in San Francisco, the Dutch team used a national registry to investigate more than 25,000 women who received IVF or other fertility treatments between 1980 and 1995. Almost 19,000 of the women had had IVF, while the other sub-fertile women had had different treatments or none.

There was no statistically significant difference in breast cancer incidence between either group as a whole and the general population. There was a slight increase in breast cancer risk among the infertility patients who had been followed up for the longest periods – 15 years – but this was accounted for by the size of their families.

Women who seek fertility treatment tend to have fewer children than average, and to start having them later in life. Breast cancer studies must always correct for family size and childbearing patterns, because women who have more children at younger ages are known to be less likely to develop the disease.

“After 15 years, the risk was a bit higher than in the general population, but this could be through a difference in the number of children compared to the general population,” Dr van den Belt-Dusebout said. “When the hazard ratios are adjusted for parity [number of children] they are not significantly different.”

The study also compared women who had had different numbers of IVF cycles, and found no relationship between extra cycles and breast cancer risk. This is important because women who have more cycles are exposed to higher amounts of drugs, and the lack of a dose-response relationship suggests there is no effect.

“Our preliminary analysis indicates that for 15 years after IVF there is no increased risk of breast cancer compared to the female general Dutch population,” Dr van den Belt-Dusebout said.”

The study also revealed no significant differences in breast cancer risk between the infertile patients who had IVF and those who did not.

The findings support the outcome of a major review of the evidence published in 2004, which also found no causative link between IVF and breast cancer. The study even found that IVF may actually decrease breast cancer risk when it is successful, by allowing women to give birth and thus to gain the protective effect of having children.

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