Most women have one breast slightly larger than the other. But checking for large assymetries could help screen for disease.© Getty

Women with very lopsided breasts may be more likely to develop breast cancer, according to a preliminary study.

Diane Scutt of the University of Liverpool, UK, and her colleagues calculated the breast volume of healthy women from mammograms taken between 1979 and 1986 as part of a larger cancer study. They selected 252 women who went on to develop breast cancer between that time and 2002, and another 252 women who remained healthy.

They found that the women who went onto develop breast cancer generally had more asymmetric breasts. If a woman's breasts differed from each other by 100 millilitres, she was 50% more likely to develop the disease than someone with symmetrical breasts.

The researchers do not know exactly why women prone to breast cancer might have lopsided breasts many years before the tissue has started to turn visibly cancerous. But they suggest that asymmetry could be a symptom of other genetic or health factors that predispose a woman to cancer.

Nobody's perfect

Before all women panic in front of the mirror, Scutt is quick to point out that most women have some small asymmetry in their breasts; only one person in the study had breasts that were a perfect match. The average difference was 50-60 millilitres for a typical 500 millilitre breast. "We don't want to be alarmist," Scutt says.

But it is possible that a quick study of breast symmetry during regular mammograms might identify those women at greater risk of developing cancer, she suggests, when weighed with other risk factors such as a family history of the condition. "Maybe it's an additional thing we should be looking at," Scutt says.

Scutt next wants to try and define the cut-off point at which a normal asymmetry becomes one that might increase the chances of cancer, and how important this is compared with other risk factors. The team reports the findings in Breast Cancer Research¹.

Sign of health

There is a theory that symmetry in body features, from ears to fingers to breasts, is a sign that a person has a good set of genes. Developing precisely symmetrical breasts at puberty, for example, may require an investment of energy and the ability to resist wayward influences, from poor diet to toxins. So perhaps only the very healthy can produce perfectly matched breasts.

Looking for asymmetry is one way to target individuals who have health problems agrees Randy Thornhill, who studies body asymmetry at the University of New Mexico, Albuquerque.

But Thornhill also emphasizes that perfect matches are very rare. "Asymmetry is the norm," he says. Symmetrical specimens "are seen only in Playboy magazine".

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• References

  1. Scutt D., Lancaster G. A.& Manning J.T. Breast Cancer Res., 8. R14 (2006).