Study objective. To assess the relative importance of heritable characteristics and lifestyle in the development of 'maternal obesity' after pregnancy. Setting. South east London, in the homes of mothers who had delivered their babies at either Guy's, Lewisham or St Thomas's hospitals. Participants. Seventy four mothers of low antenatal risk who had been enrolled in the Antenatal Care (ANC) Project (a previous trial of antenatal care) during the first trimester of pregnancy, and who had subsequently been followed up 2.5 years after delivery. Design. Information on parental obesity, psychosocial and sociodemographic factors as well as lifestyle, was gathered during a semi-structured interview at each mother's home. Additional anthropometric and psychosocial data were taken from the existing ANC Project database. These data were used to assess the relative importance of heritable characteristics and lifestyle on changes in maternal body weight from the beginning of pregnancy to the follow up interview. Main Results. After adjusting for the effects of potential confounders and known risk factors for maternal obesity, women who selected larger silhouettes to represent their biological mothers were significantly more likely to have higher long term weight gains than those who selected thinner maternal silhouettes (r = 0.083, p = 0.004). Women who were less satisfied with their bodies postpartum had significantly greater long term weight gains than those women who displayed no increase in dissatisfaction with their bodies after pregnancy (r = 0.067, p = 0.010). Conclusions. A heritable predisposition to gain weight together with changing attitudes to body size, both had an independent role in the development of maternal body weight after pregnancy. Differences in each woman's heritable predisposition to gain weight and any changes in body image that occur after pregnancy might explain why some women gain weight in association with pregnancy.