Women who went up a skirt size every decade after their mid-20s had a 33% greater risk of breast cancer after the menopause, say researchers at University College London.
Watching your skirt size from your mid-20s onwards could be a simple way to track weight gain, they told BMJ Open.
Obesity is a known risk factor for cancer, particularly midriff fat.
Prof Usha Menon of the Department of Women’s Cancer, who led the study, told BBC News: “If skirt size could be confirmed by others as a good predictor of breast cancer risk in older women, this would be a very simple and easy way to monitor weight gain.”
The study tracked more than 90,000 women in their 50s and 60s living in England.
During the three-year follow-up period, 1,090 women developed breast cancer.
The researchers found that a unit increase in UK skirt size every 10 years (for example from 12 to 14) between 25 and post-menopausal age was linked to a 33% increased risk of breast cancer.
Going up two skirt sizes in the same period was associated with a 77% greater risk, they report.
Commenting on the research, Simon Vincent of Breakthrough Breast Cancer said: “We know that 40% of breast cancers could be prevented by changes to lifestyle such as being regularly active and maintaining a healthy weight.
“This study highlights an easy way to monitor your weight gain over time. Women are more likely to remember their skirt size when they were younger than their BMI.”
The researchers said the study had some limitations – it relied on women being able to accurately recall their skirt size in their 20s.
But if the findings are confirmed, it could give women a simple and easy-to-understand message about the risks of obesity.
Tom Stansfeld of Cancer Research UK said the study could be unreliable as dress sizes had changed over the years and it relied on a woman being able to remember her skirt size several decades earlier.
“Evidence tells us the most important things you can do to reduce breast cancer risk, especially after the menopause, is to keep a healthy weight, be physically active as often as you can, and cut down on alcohol,” he said.
“Keeping a healthy weight is important to help reduce breast cancer risk after the menopause, and looking at skirt sizes to help women understand this is interesting, but knowing if you’re overweight is more important.”