Women who go up one skirt size every 10 years between their mid-20s and mid-60s are at a 33% greater risk of developing breast cancer after the menopause, new research has found.
Weight gain measured through an increase in body mass index (BMI) was known to be a risk factor, particularly around the midriff, but a study of 90,000 women is the first to make a connection between breast cancer and skirt size.
The results show that an expanding waistline could be more harmful. Those who went up by two sizes every decade in the same period saw an increased risk of 77%, according to the University College London study.
The research could provide a valuable insight into breast cancer prevention.
Usha Menon, who co-authored the report, said: "Given that obesity is now emerging as a global epidemic, from a public health perspective our findings are significant as they provide women with simple and easy to understand message.
"It needs effort to calculate the BMI from height and weight, and most of us do not remember what it might have been some years ago. In that respect, skirt size as a proxy for waist circumference is easily remembered over time."
For the study, researchers questioned 92,834 women in their 50s and 60s who were taking part in a cancer screening trial, the UK Collaborative Trial of Ovarian Cancer Screening.
The participants, not known to have breast cancer, were asked about their current and previous skirt sizes. The women's average skirt size at the age of 25 had been a UK 12, a US 8, at the beginning of the study. The average size for women aged 64 was a UK 14, a US 10.
During a three-year follow-up period, 1,090 women developed breast cancer. It was estimated that the five-year absolute risk of postmenopausal breast cancer rises from one in 61, to one in 51, with every one unit increase in skirt size every ten years.
It was found that BMI did not significantly improve the prediction risk in the study, published in the journal BMJ Open.
The study was observational, so definitive conclusions could not be made, but the researchers pointed out that expanding waist has previously been linked to ovarian and pancreatic cancers.
Simon Vincent, of the charity Breakthrough Breast Cancer, told the Telegraph: "We know that 40% of breast cancers could be prevented by changes to lifestyle such as being regularly active and maintaining a healthy weight."
He added that the study highlighted a simple way of monitoring weight gain, as a BMI is difficult to remember.
"Here at Breakthrough Breast Cancer, we encourage all women to raise their pulse and reduce their risk. Women should take part in regular physical activity of moderate intensity for 3.5 hours per week," he said.
There are limitations with the research, including that a woman may not be able to remember her skirt size from several decades earlier.