BACKGROUND: The relationship of pre-pregnancy body mass index (BMI) and gestational weight gain (GWG) with continued breastfeeding (BF) in Japanese women remains unclear. To evaluate different maternal pre-pregnancy BMI and GWG categories according to the initial BMI, we conducted an analysis of continued BF until 6 months postpartum in a large Japanese cohort. METHODS: This study used data from the Japan Environment and Children's Study, which included approximately 100,000 pregnant women. Pre-pregnancy BMI (kg/m RESULTS: Of 82,129 women with singleton pregnancies, BMI was categorized as underweight in 16.6%, normal weight in 75.3%, and overweight in 8.1%. The rate of EBF in these groups was 37.1% for underweight, 37.2% for normal weight, and 26.8% for overweight. In underweight women, both insufficient and excessive GWG were associated with higher rates of non-EBF type-II. In normal-weight women, insufficient and excessive GWG were associated with higher rates of non-EBF type-II. Overweight women showed higher rates of non-EBF type-I and type-II, irrespective of GWG. CONCLUSIONS: Our results indicate that the effects of GWG on continued BF were associated with pre-pregnancy BMI. Adequate GWG should be recommended to underweight and normal-weight women to promote BF. Efforts toward the prevention of overweight prior to pregnancy should be encouraged to enhance BF.