Pregnancy and body weight development are intertwined in complicated patterns. Of the obese patients at our Obesity Unit, 73% had retained > 10 kg in connection with a pregnancy. For the general population the effect of a pregnancy on future weight development is surprisingly difficult to predict. In the Stockholm pregnancy and weight development study the estimated mean weight retention associated with a pregnancy and estimated 1 year after delivery was 0.5 kg but ranged from -12 to +26 kg. Weight increase during pregnancy was the strongest predictor for sustained weight retention 1 year later. Pre-pregnancy weight did not predict the weight development outcome. The lactation pattern had only a minor influence on weight development. Smoke cessation was an important predictor for sustained weight increase. More weight retention was observed in those women who reported an unfavorable change in lifestyle as regarded eating habits, meal patterns and physical activity. The eventual body weight after pregnancy seems to be more determined by the changes associated with that particular pregnancy than with the lifestyle before.