The study looked at how a woman's weight before pregnancy and weight gain during pregnancy affect how long she breastfeeds in Japan. They used data from a big study with around 100,000 pregnant women. Women who were underweight or normal weight before pregnancy and gained the right amount of weight during pregnancy were more likely to breastfeed exclusively for 6 months. Women who were overweight before pregnancy were less likely to breastfeed exclusively. It's important for underweight and normal-weight women to gain the right amount of weight during pregnancy to help with breastfeeding. Preventing overweight before pregnancy can also help women breastfeed longer.