Objective: to understand postpartum weight retention among women of different body sizes at pre-pregnancy, and to explore factors influencing this weight retention. Design: a comparative, descriptive study. Pre-pregnancy body mass indices (BMIs) were determined from hospital records. Dependent variables were measured by self-report questionnaires. Setting: a 3,900-bed medical centre in northern Taiwan that delivers approximately 4,000 children each year. Participants: a systematic sample of 810 women who had delivered children at a medical centre. In total, 602 valid questionnaires were collected. Findings: the average pre-pregnancy and six month postpartum BMIs were 21.5 [standard deviation (S.D.)=3.32] and 22.48 (S.D.=3.39), respectively. Overweight (OW) and obesity (OB) increased from 18.27% before pregnancy to 27.57% at six months postpartum. Gestational weight gain (GWG) was 14.36, 14.37, 13.07 and 11.15 kg among women in the underweight (UW), normal weight (NW), OW and OB groups, respectively; and six month postpartum weight retention was 3.32, 2.57, 1.67 and -0.29 kg. GWG was the most significant predictor of postpartum weight change across groups. Key conclusions: women with different body sizes at pre-pregnancy have common and specific risks for postpartum weight retention. Implications for practice: individualised interventions should be developed to manage postpartum weight for women of different body sizes. Interventions should establish criteria for GWG and provide strategies to improve body image and modify lifestyle. The overall goal should be to manage postpartum weight gain and to promote women's health.