Breastfeeding can have lasting effects on a mother’s weight, according to a new study from the University of Oslo. Women who breastfeed for at least three months may gain up to 6.5 kilograms less over their lifetime compared with those who breastfeed less.

Previous research mainly focused on weight changes in the first one or two years after childbirth. This study, however, tracked women for decades—up to 50 years after they stopped breastfeeding—and also aims to inform research on mothers’ cardiovascular health risks.

The study analyzed data from the Women and Health Study, which included over 170,000 Norwegian women, and was published in the American Journal of Clinical Nutrition. The largest weight differences were seen in women who were overweight or obese before pregnancy. In this group, women who breastfed each child for three to 15 months gained on average up to 6.5 kilograms less from young adulthood to middle age than those who breastfed less.

For women of normal weight before pregnancy, breastfeeding for three to 15 months was associated with a reduction of up to 3 kilograms. Among women who were underweight, breastfeeding had little effect on long-term weight.

Breastfeeding increases energy expenditure, which can promote weight loss, though it may also stimulate appetite in some women. Historical patterns of breastfeeding showed the clearest long-term effects among women who had children after 1980, reflecting practices more similar to today’s mothers.

Norwegian guidelines recommend partial breastfeeding for at least one year if both mother and child are comfortable. The study highlights the importance of supporting breastfeeding, particularly for women with overweight or obesity, to help improve long-term health outcomes.