Breastfeeding may help mothers stay in shape decades after they have given birth, research suggests.
A study of 740,000 post-menopausal women in the UK found that childbirth and breastfeeding have significant, but opposite, effects on long-term weight.
The more children a woman had, the higher her body mass index (BMI) decades later, scientists from Oxford University found.
But they also found that the average long-term BMI was significantly lower in women who breastfed than in those who did not, regardless of how many children they had.
For every six months of breastfeeding, their BMI was one per cent lower, even after taking into account other factors known to influence obesity risk such as smoking, exercise and social deprivation.
The study which was funded by the Medical Research Council and Cancer Research UK, was published in the International Journal of Obesity.
The researchers analysed data from the Million Women Study, an ongoing investigation into how reproductive and lifestyle factors affects women's health.
Women taking part in the study had an average age of 57 and BMI of 26, regarded as being slightly overweight.
BMI is calculated by taking a person's weight in kilograms and dividing it by their height in metres squared. A BMI or more is classed as overweight, while a BMI of 30 or more is classed as obese.
Of the women taking part in the study, 88 per cent had had at least one child and 70 per cent of these had breastfed for an average of 7.7 months.
Previous research had shown that breastfeeding can help women lose the weight they gained during pregnancy in the months immediately after birth. But fewer studies have looked at the effect of breastfeeding on long-term BMI, the researchers said.
Professor Dame Valerie Beral, Director of the Cancer Epidemiology Unit at the University of Oxford and study co-author, said: "Our research suggests that just six months of breastfeeding by UK women could reduce their risk of obesity in later life.
"A one per cent reduction in BMI may seem small, but spread across the population of the UK that could mean about 10,000 fewer premature deaths per decade from obesity-related conditions, such as diabetes, heart disease and some cancers."
Dr Kirsty Bobrow, also from the University of Oxford and lead author of the paper, said: "We already know breastfeeding is best for babies, and this study adds to a growing body of evidence that the benefits extend to the mother as well – even 30 years after she's given birth.
"Pregnant women should be made aware of these benefits to help them make an informed choice about infant feeding."