Published online 26 September 2005 | Nature | doi:10.1038/news050926-3

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Exposure to chemicals in the womb may explain link.

The hand you write with may be a telltale sign of exposure to hormones in the womb. © Punchstock

Left-handed women face double the risk of developing breast cancer before the menopause compared with right-handed women, according to a new study.

This sounds like a strange coincidence. But researchers who have just completed a study on the odd association say that exposure to hormone-like chemicals in the womb may be to blame for both. Left-handedness in some people, they say, may be an effect of prenatal factors that could also lead to cancer.

Exactly what causes some people to prefer using their left hand remains largely unknown. Researchers have pointed to genetics and early fetal development. At least one study, for example, has indicated that prenatal exposure to the oestrogen-like chemical diethylstilbestrol increases a woman's chance of being left-handed1. But the full mechanism behind handedness remains elusive.

Some scientists think that clues to breast cancer also lie in the prenatal period of human development. Studies in mice have found that fetal exposure to oestrogen-like compounds can increase the risk of breast or prostate cancer in later life.

Now, researchers in the Netherlands say these two links to oestrogen-like chemicals could be creating an association between handedness and cancer.

Forward planning

Cuno Uiterwaal of the University Medical Centre Utrecht and his colleagues examined a medical database for thousands of healthy middle-aged women born between 1932 and 1941. All of the women selected for the research had an unknown risk of cancer from the start of the data-collection process, which lasted 16 years. This kind of forward-looking study yields the soundest statistics in studies of health.

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After examining the records of over 12,000 women, Uiterwaal's team found that left-handed pre-menopausal women face double the risk of developing breast cancer as right-handed women do. This trend did not apply, however, to women who had not given birth or to those who were overweight or obese. The team reports the results in the British Medical Journal2.

It is not the first study to focus on this association. Other, retrospective studies have found a similar link between handedness and breast tumours. But the Dutch team thinks their thorough approach lends more support to the idea that the effect is real, and that chemicals in the womb could be to blame.

In some countries, bans are in place to prevent oestrogen-like chemicals from being used in plastics. Others are campaigning to reduce the amount of such chemicals flooding into our environment from contraceptives and other sources.