For some women pregnancy is a trigger for developing overweight and obesity. Seventy-three per cent of 128 female patients at our Obesity Unit indicated that they had retained more than 10 kg after each of their pregnancies, and for this subgroup weight development after pregnancy was of crucial importance for their future health. Although mean weight increases after pregnancy generally are modest, there are wide individual variations. In studies at the Obesity Unit, weight retention ranging from up to 26.5 kg one year after pregnancy to a loss of 12.3 kg was reported, although the mean weight gained was only 0.5 kg. Numerous studies have analysed factors explaining weight development after pregnancy and delivery, with a range of subjects from several hundred thousand women to fewer than one-hundred, but overall it has been surprisingly difficult to identify strong predictors of weight development. Numerous confounders have been identified; in a review up to 31 such confounders were reported. Methodological problems include weight development over time also in non-pregnant women and problems of identifying the optimal time-point when the overall impact of the pregnancy on weight development should be evaluated. Lactation has consistently been found to play a small role in explaining weight retention up to one year after delivery. Few studies have examined the role of physical activity during pregnancy and after delivery to explain weight development. Our own ongoing follow-up of women who gave birth during 1984-85, the so-called SPAWN (Stockholm Pregnancy and Women's Nutrition) study, illustrates that 15 years after delivery, a significant proportion of the 1423 women initially studied were available for re-examination. Drop-out analyses indicate that for most variables under study, the remaining women were representative for the initial sample. Pregnancy and weight development are intertwined in a complex pattern, which includes a change in lifestyle factors, such as eating behaviour, physical activity, smoking cessation and degree of lactation, but which are still not fully understood.