Women who go up a skirt size over a 10-year period between their 20s and 60s have a bigger risk of developing breast cancer after the menopause, a new study has found.

According to researchers in the UK, while it is already known that weight gain during adulthood can be a risk factor for breast cancer, gaining weight in the waist area appears to be especially harmful.

They looked at almost 93,000 women over the age of 50 who had already gone through the menopause. None of the women had breast cancer at the start of the study.

All of the participants provided detailed information on their health, including their body mass index (BMI), family history of breast cancer, reproductive history and their use of HRT. They were also asked what skirt size they had been in their 20s and what shirt size they were now.

The women were then monitored for up to four years, at the end of which they were again asked about their health, including whether they had been diagnosed with breast cancer.

Almost 1,100 of the women developed cancer during the study period. After taking into account risk factors for the disease, such as family history and use of HRT, the study found that an increase in skirt size was the strongest predictor of breast cancer risk.

In fact, going up a skirt size over a 10-year period increased the risk of developing breast cancer after the menopause by 33%. Going up two skirt sizes over a 10-year period was linked with a 77% increased risk.

The researchers noted that three out of four of the women who took part recorded an increase in their skirt size during adulthood. In their mid-20s the average dress size was a 12. At the start of the study, when the women had an average age of 64, the average dress size was a 14.

"Although the exact mechanism of these relationships need to be better understood, there is a suggestion that body fat around the waist is more metabolically active than adipose tissue elsewhere," the team from University College London said.

They added that excess fat is known to boost levels of the female hormone oestrogen, which many breast cancer cells rely on for fuel.

Details of these findings are published in the journal, BMJ Open.