Breastfeeding may help women keep their weight in check and make them less likely to become obese as they get older.

A large new study shows postmenopausal women who breastfed for at least six months had a lower body mass index (BMI) than those who did not, regardless of how many children they had.

The longer women breastfed, the bigger the benefits.

Researchers found for every six months of breastfeeding, women's average BMI was 1% lower than that of women who did not breastfeed.

Although that may not seem like much, researchers say it could have a major impact on women's long-term health and risk of obesity.

"Even a modest 1% reduction in BMI would substantially reduce the number of obesity-related diseases and their costs," write researcher Kirsty Bobrow, of the University of Oxford, and colleagues in the International Journal of Obesity.

"It seems relevant to inform women that breastfeeding is associated with a relatively small, but important, persistent reduction in their weight decades later," they write.

Breastfeeding Hints and Hurdles
New Breastfeeding Benefit

Bobrow's team looked at the long-term effects of women's childbearing patterns on their BMI. More than 740,000 postmenopausal women in the U.K. participated in the study and reported their height, weight, and reproductive history.

Researchers divided the women into groups based on the number of children they had given birth to and their breastfeeding history.

The results showed that the women's average BMI increased with the number of children they had, from an average of 25.6 among women without children to 27.2 for women with four or more children.

A healthy BMI ranges from 18.5 to 24.9. A BMI of 25-29.9 is considered overweight, and 30 or higher is obese.

Among women with children, 70% said they had breastfed. The average duration of breastfeeding was almost eight months.

The average BMI was lower among women who had breastfed, regardless of the number of children they had.

Specifically, the average BMI decreased by about 1%, or about a half a pound, for every six months of breastfeeding.

This weight-control benefit of breastfeeding also persisted after taking other risk factors for obesity into consideration, such as smoking, physical activity, and socioeconomic status.

This additional long-term benefit of breastfeeding may seem small. But researchers say earlier studies have linked a similar drop in BMI to a 1% drop in deaths from any cause.