Many mothers have long believed that breastfeeding helps them get their figure back after giving birth.
Now scientists have found it can help them stay slim for decades.
They discovered women who breastfed their babies, even for a few months after the birth, were less likely to be obese 30 years later.
The Oxford University researchers worked out that for every six months a woman gives her baby breast milk, she loses around 0.9kg - depending on her initial weight and height.
Although this may not sound much, the academics claim that it could help prevent thousands of deaths from cancer, heart problems and other illnesses related to obesity.
Doctors and midwives are growing increasingly concerned many women are becoming obese by gaining weight during pregnancy, which they never manage to shift after the birth.
If they go on to have several children, they will become progressively fatter with every pregnancy.
The latest study - involving more than 740,000 women - provides compelling evidence breastfeeding could not only help reverse this weight gain, but help women keep the weight off for good.
Breastfeeding uses up large amounts of energy. Experts have previously calculated it burns 500 calories a day - the same as a typical gym session.
Now academics have worked out that for every six months a woman breastfeeds her baby, her body mass index - the measurement of obesity - falls by one per cent.
An average woman who is 5-foot-6-inches tall and weighs 70.3kg would have a BMI measurement of 25, which is classified as overweight.
But if her BMI was to fall by one per cent it would be about 24.75 - which is deemed 'healthy' - and she would weigh 69.39kg.
The lead author of the paper, Dr Kirsty Bobrow, from Oxford University, 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." The researchers also point out that breastfeeding may help prevent thousands of deaths related to obesity from cancer, heart disease and diabetes.
Professor Dame Valerie Beral, director of the Cancer Epidemiology Unit at Oxford University, who was also involved in the study, 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."
The study, published in the International Journal of Obesity, surveyed 740,600 women whose average age was 57. They had all answered questions about how many children they had and the total number of months they had spent breastfeeding.
The research also found that women were more likely to be obese if they had several children.
This backs up the concerns of health professionals that many gain weight during pregnancy which they don't lose after the birth of their babies.
According to the latest figures from the UK's Department of Health, almost three quarters of new mothers start their babies off on breast milk.
This is far higher than the six in 10 who breastfed in the 1990s and the rise follows numerous 'breast is best' campaigns.
The World Health Organisation recommends that women breastfeed their babies for at least six months but many give up because they find it uncomfortable or are worried that their babies are struggling to get enough milk.
- Daily Mail