Fat around the abdomen may be key
A study suggests apple-shaped women with a waist bigger than 88cm (35in) have a higher risk of developing asthma - even if their body weight is normal.
Being overweight is well known to raise the risk of asthma.
But the latest study suggests that the amount of weight women carry around the abdomen might be particularly important.
The study, by the Northern California Cancer Center at Berkeley, appears in the journal Thorax.
The researchers analysed data on 88,304 female teachers and school administrators.
BODY MASS INDEX
Calculated by dividing weight in kilograms by height in metres squared
Normal: 18.5 - 24.9
Overweight: 25 - 29.9
Obese: Above 30
The results showed that overweight women were 40% more likely to have asthma than women of a normal weight.
Asthma was more than twice as likely in obese women, and more than three times as likely in extremely obese women than in those of normal body weight.
But perhaps more surprisingly, the researchers also found that women of normal body weight, but with a waist circumference of more than 88cm were also at increased risk - around a third higher than those with a smaller waist.
Overall, 5.4m people in the UK have asthma, and rates have been rising in recent years.
Body mass index (BMI) has been widely used as a standard measure of obesity.
But some researchers argue that waist circumference may be a more useful measure because it more closely reflects levels of visceral fat deposits found around the body's organs.
Visceral fat is metabolically different from other types of fat found in the body, and may have different - and more profound - effects on health.
Researcher Julie Von Behren said: "Visceral fat is metabolically more active - it can produce compounds that may cause inflammation. Inflammation may then be related to asthma."
Leanne Metcalf, director of research at the charity Asthma UK, said the researchers had not taken into account other factors which might have influenced the development of asthma, such as a family history of the condition.
However, she said: "As this study involves such a large number of women, the results are likely to have some significance.
"It is important that people with asthma eat a healthy, balanced diet, which is low in fat and sugar, and take regular exercise.
"Taking these steps can aid weight loss, improve lung function and help get asthma symptoms under control, especially given the established links between high levels of abdominal fat and other long-term health conditions."
Dr Noemi Eiser, of the British Lung Foundation, agreed that the study reinforced the need for women to lead a healthy lifestyle and for them to be more aware of the health risks of having a large waist size.