The aim of this study was to investigate whether the birthweight of Scandinavian children varies according to the social class of their parents, especially the mother. Data were taken from the Scandinavian part of an international multicentre study of fetal growth and perinatal outcome. The occupations of the pregnant woman, her spouse and her parents were registered according to the Nordic classification of occupations. This classification has been criticised for being too detailed to be suitable in epidemiological studies, and the data were recorded into the British system of five classes. The birthweight of female newborns in social class V was 301 g lower than in the other social classes (p < 0.05). A corresponding difference was not shown among male newborns. Newborns of women that had migrated downwards in the socioeconomic system, were 117 g lower than if the migration was upwards (p < 0.05). This difference among female newborns was 164 g (p < 0.05). This study demonstrated that there are differences in birthweight according to social class. This may partly be due to genetic factors and a higher prevalence of smoking and high body mass index, i.e. a less favourable lifestyle in the lower social classes.