These findings come from personal health questionnaires completed by more than 90,000 postmenopausal women, all of whom were taking part in a cancer screening trial. Of those 90,000+ women, roughly 1,000 developed breast cancer, according to the UK study team. After adjusting for other risk factors like family history of cancer, the study team linked a growing waist circumference to a greater risk for the disease. MORE: The Breast Cancer-Fighting Fruit For women who went up two skirt sizes per decade, the picture gets bleaker: A two-size increase increased their breast cancer risk by 77% compared to women who did not experience this level of mid-body weight gain. To be clear: If you’re 50 or 60 and don’t fit into those skirts your wore back in your 20s, that’s not reason to worry. But if you wore a size 6 during your 20s and now, at age 55, you wear a size 12, you’d fall into the elevated risk category the study identified. What does your skirt size have to do with your breast cancer risk? Research is piling up that suggests body weight, and especially weight around your midsection, can contribute to the development of cancer. While the exact mechanisms aren’t well understood, more fat in your midsection has been linked to higher levels of certain sex hormones (like estrogen) and increased levels of inflammation—both of which could explain the elevated cancer rates, according to study coauthor Usha Menon, PhD, head of the Gynecological Cancer Research Centre at University College London.   Another possibility: “Previous studies suggest that body fat around the waist is metabolically more active than fat tissue elsewhere in the body,” say Menon and colleagues. And research shows that uptick in metabolic activity appears to promote the spread and aggressive nature of some cancer cells. If you’re worried about your breast cancer risk, it makes sense that staying fit—especially around your midsection—could help keep you out of harm’s way. While Menon says her study didn’t look at this specifically, other studies have linked higher rates of exercise to a reduced risk for breast cancer. Avoiding a high-fat diet may also help lower your risk, research shows. MORE: 12 Breast Cancer Myths You Should Ignore