Women with a waist larger than 88cm face an increased risk of early death – even if they have a "healthy" weight, researchers have warned.
It means those with a dress size 18 or above could be in danger, even if they have always been told they do not need to worry about their weight.
The research found middle-aged women with a waist exceeding 88cm had a 30% raised risk of early death.
The findings cast into doubt the validity of body mass index – the standard measure of whether someone is of a healthy weight. Experts stress that BMI, which is calculated by dividing weight by the square of your height, does not differentiate between muscle and fat.
Neither does it tell doctors where fat is accumulated in the body. Fat around the waist is dangerous as it wraps around internal organs.
Professor Wei Bao, who led the research at the University of Iowa in the US, said: "The results suggest we should encourage physicians to look not only at body weight but also body shape when assessing a patient’s health risks."
The findings are particularly alarming as the average waistline of women in England now exceeds 90cm. They have grown an average of 7.6cm in 24 years, from 82cm in 1993 to 89.4cm in 2017, NHS Digital figures show.
The research tracked 157 000 post-menopausal women in the US aged 50 to 79 from 1993 to 2017.
It found those who had a "healthy" BMI – between 18.5 and 25 – but a waist size of more than 88cm were 31% more likely to die during the study than those who had a similar BMI and slimmer waist.
They were most likely to die of heart disease or obesity-related cancer. Professor Bao said: ‘People with normal weight based on BMI, regardless of their central obesity, were generally considered normal in clinical practice according to current guidelines.
"This could lead to a missed opportunity for risk evaluation and intervention programmes in this high-risk subgroup."
British experts last night welcomed the findings, which were published in the JAMA Network Open medical journal.
Dr Katarina Kos, of the University of Exeter, said: "The authors have an important message, which is for everybody keeping up a healthy lifestyle regardless of our overall build."