WOMEN who have IVF in their twenties are up to 50 per cent more likely to develop breast cancer, a study has found.
Across all age groups there was no overall increased risk of breast cancer associated with IVF.
However researchers at the University of Western Australia found women who had IVF treatment at a younger age had a higher risk of developing the disease over the following 15 years.
Oestrogen, a hormone which is known to fuel certain forms of breast cancer, may be up to 13 times higher during an IVF cycle than normal.
Concerns have been raised about the use of strong hormones to stimulate the ovaries to produce more eggs during fertility treatment as hyperstimulation can be fatal.
The study compared women having IVF treatment with those having treatment for infertility but not having IVF and with the general female population.
There were 21,025 women in the study who were followed up for an average of 16 years. Of those 7,381 had IVF treatment and 384 women developed breast cancer.
Women who have IVF treatment are more likely to have babies at a later age than the general population and this is known to increase the risk of breast cancer. This was taken into account in the findings.
Half of the women had only one or two cycles of IVF treatment and it is not known what doses of drugs they received.
Women starting their infertility treatment at age 24 were 1.5 times more likely to develop breast cancer than women of the same age not having treatment.
The researchers wrote in the journal Fertility and Sterility: "There was an increased rate of breast cancer in women who commenced IVF at a young age, but no positive association between IVF and breast cancer in those who delayed treatment."
It is not known how many women started fertility treatment at age 24 in Britain however four in ten are between the ages of 18 and 34, according to data from the Human Fertilisation and Embryology Authority, the fertility regulator.
Independent experts warned that this could be a chance finding, or be a short-term increased risk similar to pregnancy itself which resolves over a lifetime.
It was added that if it were a real increased risk it would would be the equivalent of two extra cases of cancer over 15 years per 1,000 women.
The research also confirmed the link between the age women have their first child and breast cancer risk with a two fold increased risk in those who first gave birth after age 35 compared with those younger than 25.
It was also found that women having twins or triplets were at lower risk of breast cancer.
Lead author Louise Stewart of the School of Public Health at University of Western Australia, wrote: "The results of this study will be reassuring to women who commence IVF treatment in their thirties and forties, because for these women, there appears to be no direct association between IVF treatment and breast cancer risk.
"Nevertheless, women should be aware that delivering their first child late in reproductive life, whether assisted by IVF or not, is associated with an increased risk of breast cancer.
"For younger women there is some cause for concern, because it appears that they may face an increased risk of breast cancer after IVF treatment.
"If the veracity of this result stands up to critical review and confirmation, it will be an important part of the process of informed consent for younger women commencing IVF to appreciate the trade-off in lifetime objectives and risks that they may face."
Dr Linda Giudice, President-elect of the American Society of Reproductive Medicine, said: “The development of breast cancer is linked to oestrogen exposure and the longer one is exposed, the greater the risk.
"In an IVF cycle, there is a short, but significant elevation in circulating oestrogen, and whether this is linked to the observations found in this study is not clear at this time.
"Women should be reassured that, overall, IVF was not associated with an increased risk for development of breast cancer.
"However, as noted in the study, women in their thirties and forties still need to be aware of the increased risk of breast cancer associated with delivering one’s first child at this stage of reproductive life.
"For younger women, there is the possibility that IVF s associated with increased risk, but more research is needed to confirm this.”
Dr Michael Jones, an epidemiologist from The Institute of Cancer Research, said: “This is not a randomised-controlled trial, so although the authors have tried to minimise variables that could influence their results, there may be other factors that explain the results that they have not been able to control.
"A lot more work would be needed before we can say definitively whether these results are true or not.
“The finding that women who first underwent IVF at the age of 24 were 50 per cent more likely to develop breast cancer looks worrying, however this must be seen in the context of the very low rates of breast cancer among these women. A 50 per cent increase on low would still be low.
“It is also not clear from this study whether this increased risk continues over their lifetime or is just temporary. Breast cancer rates increase with age, so if this risk factor continued for life then it would ultimately be quite a large increased risk.
"However the study ran for only 16 years so it is equally possible these women’s rates of breast cancer return to normal levels in later life.
“Importantly, the study also found that there was no overall increase in breast cancer rates among women who had IVF.”
Dr Paul Pharoah, Reader in Cancer Epidemiology, University of Cambridge, said: “The findings may be the result of chance or bias. If real, the absolute increase in risk is small and should not be regarded by women considering IVF as relevant. The effects of pregnancy on breast cancer risk occur over several decades, and this study provides no data on the long term effects of IVF on breast cancer risk."