Childbirth and breastfeeding have significant, but opposite, effects on long-term weight, according to a study of 740,000 post-menopausal women.
Scientists found the more children a woman had, the heavier she was likely to be in later life. But average body mass index (BMI), a standard measurement relating weight and height, was lower in women who breastfed, irrespective of how many times they had given birth.
Every six months of breastfeeding reduced a woman's long-term BMI by 1%, the research showed. This was after taking account of factors known to influence obesity risk, such as smoking, exercise and social deprivation.
Professor Dame Valerie Beral, director of the Cancer Epidemiology Unit at Oxford University, a member of the study team, said: "Our research suggests that just six months of breastfeeding by UK women could reduce their risk of obesity in later life.
"A 1% reduction in BMI may seem small, but spread across the population of the UK, it could mean about 10,000 fewer premature deaths per decade from obesity-related conditions, such as diabetes, heart disease and some cancers."
The study, which was funded by Cancer Research UK and the Medical Research Council, is reported in the International Journal of Obesity.
Professor Dame Sally Macintyre, director of the MRC/Chief Scientist Office Social and Public Health Sciences Unit, based at Glasgow University, said: "The obesity epidemic is one of the biggest challenges facing both high-income and, increasingly, low and middle-income countries.
"Rates of obesity are continuing to rise. Studies such as this one, which look at broad trends within a large population, can help us to develop effective strategies to prevent obesity and its related diseases."
The research formed part of the Million Women Study, a major investigation into reproductive and lifestyle factors affecting women's health.
Participants had an average age of 57.5 and BMI of 26.2, which is classified as "overweight". A BMI of 30 marks the point at which an overweight person becomes obese.
Most of the women had given birth to at least one child and of these, 70% had breastfed for an average 7.7 months.
Previous research had shown that breastfeeding can help women lose the weight they put on during pregnancy in the months immediately after birth. However, the long-term impact of breastfeeding was unclear.
Lead author Dr Kirsty Bobrow, also 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."
Sara Hiom, from Cancer Research UK, said: "We already know breastfeeding can reduce a woman's risk of developing breast cancer.
"And this study highlights breastfeeding may also be linked to weight. Weight in turn influences the likelihood of developing some cancers as well as other diseases."
She added: "Too few people know about the significant cancer risks associated with being very overweight."